you rb30 pusher you..
or slap in a perfectly good rb25 bolted together.. loom, etc. Nothing to worry about drilling blocks, speaking to engine builders, etc. Good for a nice 230-250rwkw with revability. If it blows, you have all the required parts ready to go for rb30 later on..
rb20det doesnt run VCT, rb25det runs VCT (edit: oops).. you would have to disable the VCT via playing with the engine and dick around... spoils the point of the drop in approach.
I can think of quite a few parts also that are different between rb25 and rb20 looms that would cause you problems with rb20 loom: cas, ignitor, injectors. But maybe its possible.