This thread has been made to store all the tips, tricks, and how-to's on the Legacy Twin Turbo, to single turbo conversion. The following content is based on my conversion, with some research from various forums, and other people who have done the conversion before me, and since me. This thread is specifically aimed at the B and C revisions of the BG5/BD5 Legacy, that have the EJ20R motor. However this doesn't stop you from gaining a bit of knowledge on the how to's of this conversion. I ask you to please read the entire thread through before getting started too. I will be assuming you will be lifting your engine for this, and assuming you know how. It will make life a whole lot more easy for you. And lastly, this is only a rough guide, with a few pointers in it, not a manual or complete instruction set. Please remember that! If you get stuck, figure it out!
And from hereafter I'll be using "V3/4" to simple refer to Version 3 and 4 WRX's, and STi's, and Foresters with the same engine (EJ20K/EJ20G).
First thing you'll need is a shopping list, expect to pay between $1000 - $2000+ for this, but with luck, that is ALL you'll spend. I've tried to list every last washer and rubber pipe and bottle of fluid, so you don't get any hidden costs, and can run a tight budget. Item's marked with an * are not required, but do make things go a lot easier, or make your engine bay tidier etc. I strongly recommend getting everything on the list, and you will see more about them in the thread.
This list applies to the EJ20R conversion:
A Version 3/4 STi ECU
A turbo. Such as a VF22, 24, 30 etc. From a single turbo Subaru.
Single turbo Coolant supply and return lines. You can make your own
Single turbo Oil return line (rubber pipe)(Buy this new)
Single turbo Oil return line (Metal pipe in block) This will be one hard SoB to find, but you need it. Read stage 3 to find out more. (This pipe, to replace this pipe)
Single turbo exhaust manifold. A 2.5L equal length one works too, and offers better fuel economy. But you won't get that boxer rumble

Up-pipe
Downpipe.
Single turbo gear box cross member. Needed if you get a bolt on DP, not if you make your own downpipe.
Single turbo up-pipe bracket. (the one by the turbo)
Single turbo intercooler Y pipe. Anything from V3/4 and 5/6 will work
If you get an STi intercooler with your y pipe, It must be a V3/4 intercooler, and you need to find a V3/4 intercooler bracket, they are damn hard to find!
Turbo outlet to Y pipe hose (Part of Y-Pipe normally)
*V3/4 Intake manifold, complete. If you do not get a complete one, here is what you'll need for it:
*Idle control valve pipe (metal thing that goes on top of the manifold)
*BOV
*BOV Return line/rockercover breather lines (Don't forget the little rubber hoses that connect it to the rockercovers!)(it's a metal pipe hidden on the underside of the manifold)
V3/4 Turbo Inlet/intake pipe
Crank case breather line (I had to make my own)
*F-Pipe, for the crank case breather
*Right hand fuel rail if you want to do the split fuel rail, more on that later
V3-5 Heater core pipes. These pipes go from the water pump, up the side of the head, across the top of the engine, and up to the clutch housing, made of metal pipes, and joining rubber pipes.
V3-5 Heater core hoses, these go from the pipes by the clutch housing, to the firewall.
2 meters of vac line. You can make do with the leftovers from the TT system however. Budget for this anyway
*Single turbo solenoid bracket (This is so you can replace the BBoD)
*Pressure select solenoid (a 3 port solenoid from a single turbo, this is only to tidy up the wiring, you actually already have one in the BBoD that you can use - but it has a great big plug with like 15 wires.)
Bolts to block off the primary turbo's Oil supply, return and coolant return lines on the left hand head. You can pick these off N/A's are Pick-A-Part!
Gaskets for:
Intake manifold
Throttle body
Exhaust manifold
Turbo/up-pipe
Turbo/downpipe
Coolant
Oil
Oil filter + Sump plug washer
And Budget an extra $200 for crane hire, and tools you don't have etc.
If you wish to do the split fuel rail mod while you're at it (estimated cost, $150), which I recommend you do since the intake manifold will be coming off anyway, all you need is
8 feet of EFI (Electronic Fuel Injection) hose. 8 ft really is just enough.
A few hose clamps (6 should do it, you can reuse the originals for the rest)
2 T sections
A second right hand fuel rail, with luck you picked one up with the intake manifold
Coil pack mod (Recommended!!!):
V3/4 Coil pack (And loom plugs), or one from a BG N/A
V3/4 Leads
V3/4 Ignitor (And loom plugs), or one from a BG N/A
New Loom (as in the actual stuff you put around wires)
Some wire for making new loom coil pack loom
For other models:
For BG5A owners - This list applies to you also, only, you probably need to run a WRX ecu, not an STi ECU.
For BE/BH owners, of the Rev A, B and C variety, you can pretty must use this list too, but the ECU, you instead use a V5 or 6 WRX ecu, NOT AN STi ECU! The STi ECU has trouble dealing with the higher compression, so you either need to run a WRX ECU, an aftermarket ECU, or lower the compression. Also, anything marked as V3/4, instead buy V5/6. So want an STi TMIC, it needs to be V5/6. Intake manifold? V5/6. Turbo intake? V5/6 etc.
For BE/BH Rev D owners... I have no idea. The Rev D is a special beast. It has it's own loom and ECU, and engine. It's internally a phase-3 engine, like in the V7 and up, but on the outside looks much the same as a phase 2. For example, it has the old intake manifold and intercooler setup, yet it runs pink injectors. Theoretically, the mechanical side of things I imagine will be much the same, as in, V5/6 intercooler etc. But I don't know about the intake manifold, and if it has different mounts for the top feed fuel rails etc. It might end up being a curious mix of V5/6 parts, and V7 parts to make it work, and it's largely uncharted territory! Certain members are currently researching the conversion, and I'm sure more information will come to light!