I've looked at the fab jobs for doing the 20b install in my FC, it's pricey but if you do a lot of the work yourself it's just a matter of time, that's the majority of the bitch of working on it. Having someone else do it for you is the kicker part, im going to end up building my FC up to a certain point then selling it to get an FD come sometime next year or this fall/winter. Spending the cash on an FD just makes a lot more sense then it does on my FC, better looking, stronger platform, etc. More expensive tho but if it's for the car you want meh whatever.
If you're wanting to do that kind of swap, check out everything involved, talk to the shops, people in forums, rx-7club urks the shit outta me, i like nopistons a lot better but that's just me. If you post over in the 3rd gen area there they can give you the low-down since people do all types of swaps in that area.