Like you I just had to have a big block although my small block was just perfect. My motor mounts stayed in the same location only not as tall (about 1 1/2") shorter. Had to replace my headers, transmission, starter, remote
oil filter and routing of required lines, and the thing you will really love is the need for a hydraulic throw-out bearing.
Now I have this big a$$ engine that will absolutely climb trees but I can't hardly work on (especially on the drivers side when you need to change spark plugs you will find the steering column is in the way as will be your brake lines, and reservior for your throw-out bearing. Also, many times the FE engine runs a lot warmer than a small block. If you live in a city with long stop lights (eg: Dallas) you may find yourself getting nervous on thos 100 degree days.
Now the good news is you will have unbelievable torque but unless you go with a TKO II or some other trans w/ overdrive your top end will be considerably less because w/ a toploader and 3.50 or 3.73 gears you will be turning about 4000 rpm at 70 mph. Highway cruising at 4000 or 5000 rpm's make me nervous over long periods of time. But if you intend to drive your car at road races or drags you will love it. Coming out of a turn you will be able to launch down a straight away extremely fast.
Everything is a tradeoff and it all depends on what you want your car to do or look like. If I had overs I would go with a 408 stroker and a TKO II. Parts are a lot cheaper and easier to get. Just my opinion.
Just be happy with what you have.
Clois