off of the top of my head.

you can use the rb20 tranny. IIRC it just isn't quite as beefy as the rb25 tranny. but it should hold up for a while.

the mounts should be the same. just swap the rb20 mount brackets onto the rb25 block and it should drop right in.
i don't remember if the clearance issues change between the two or not. Might need to do some engine-to-hood clearance fiddling to get it right.

beyond that, it is just going to be a normal engine swap. pretty sure any modifications/fab work that would be needed is already done once you got the rb20 in it.

again, this is just from what I remember off of the top of my head, and i've personally never done this swap, so take it with a grain of salt.