Followup, because there is NEVER followup on Frankenmotor discussions. Note that when I started this thread, the concept was theoretical. Then the EJ22 done gone blowed up in the middle of Utah so I had to do something. And I had this EJ25 bottom end already and a trashed EJ22 bottom end.
Short version: the engine is in the Vanagon and working well. It is almost literally a 2.5 short block with everything else from the 2.2.
Long version, assuming I remember all the details. This was a very long project because I moved in the middle of it and the machine shop had some parts for a year and well, too many cars.
I ended up pulling apart the 2.5 short block which is a good thing because someone had pulled the pin on the Subaru hand grenade. It was about to go, the only reason it didn't suffer a catastrophic failure was because a broken accessory belt took out the timing belt and bent the valves first. So I rebuilt the bottom end, which means I took the block and replaced everything inside. It's all OE Subaru, but now it's running STI stuff because why not, it's no more expensive. The crank was damaged and a new one was less expensive than fixing the old one. Pistons were tired so they got changed, the rods were swapped because I had no confidence in the take-outs from the 2.5 - or maybe they were damaged, I don't recall. I do recall that OE Subaru parts are pretty cheap compared to forged Miata parts :) So it's got a new crank along with pistons, rods and the oil pump and the other usual stuff. Almost everything is normal EJ25 in the bottom end other than the tensioner. Or is it a middle end on a Subaru?
Heads are the SOHC EJ22 units cleaned up. The machine shop made disparaging noises when I dropped them off because blah blah 600 hp turbo and who wants EJ22 heads? Well, I do, shut up and do the work I'm paying you for. They are unmodified.
Head gasket is the MLS Frankenmotor gasket available from Cometic, part H1631SPK051S.
Timing belt kit was EJ25 for the 1998-99 Legacy, which IIRC matches the SOHC heads and the other generational parts from the short block. I did swap over the timing belt tensioner bracket from the SOHC EJ22 on to my DOHC EJ25 block and used the old-school tensioner instead of the newer design. I suspect I could have sourced a newer bracket from a SOHC EJ25 but I was trying to build all this with the parts I had on hand. There were some misadventures while installing the timing belt after repairing a poorly installed cam seal because I'm a bit of an idiot, but I got through it.
Intake manifold is the EJ22 unit because that was the easiest way to deal with the fact that the car already had EJ22 engine management. That was an important factor in the Frankenmotor decision, going to a 6 cylinder would have involved trying to reverse-engineer a swapped wiring harness done by someone else and reswapping it. I do have the EJ25 electronics on hand in case I decide to update, but I don't have a strong reason why.
End result: A very perky engine. It's not a 600 hp turbo monster but it's perfect for the use case with lots of torque right where I need it in normal driving. It is noticeably stronger than the old stock 2.2. On the highway, I actually have to pay attention to keep it below 80 when cruising at 5000' above sea level. Fuel efficiency does not appear to have changed, although I feed it premium now due to the roughly 11:1 compression. And I recently discovered that the throttle cable lash-up doesn't even allow me to reach WOT, so I have a power upgrade in my future :)
Picture of the almost-assembled setup. I note now that I have the crank in the wrong position for setting the timing belt, foreshadowing there...