You can fudge some combos together to get by but in the end unnecessary risk might outweigh reward.
The head gaskets have to match the BLOCK.
The block has to have the right number of bolts connecting engine to transmission. Autos can mate to 4 bolt engines but manuals don't have stud for the lower starter to tighten down if you use a 4 bolt main block in an 8 bolt cars transmission.
The power is going to be horrible on a 2.2 with 2.5 heads but as you figured out already the intake has to match the heads.
It might be better in the long run to sell the Outback needing an engine, get a 1000 or so out of it and get an older model that one of your engines is intended to go into instead and fix a car with its corresponding parts.
If it is just knocking take it apart, replace the crank and fix the motor instead.
Good luck and hope it all goes well.