Thanks, everything happens for a reason. Sorry in advance for the solemn stuff.
It just gets back to being honest with yourself about what you want the car to be. If you don't want/need 500hp then your build can be much milder and less expensive. It just depends on your goals. A 6.0 with a cam and headers is somewhere in the 400-450hp range which is not difficult to accomplish with a BBC. Keep in mind once you get above 350hp or so you have to start helping the chassis/rear end to be able to survive at higher power levels. You already mentioned pulling the frame to clean it up which will give you a chance to stiffen it up and replace the body bushings so you are already well on your way. My suggestion is now that it sounds like you've decided on a big block, now work on deciding on what power level you want so you can plan and budget for it.
The 8.5" 10-bolt from a 442/Hurst Olds/Grand National is strong and is a drop in. They are a little hard to come by though so they command some price. If you are going 450+ HP, you should probably look into building up a Ford 9" for it. There are new steel housings available that have all the right brackets on them and will bolt in. At those HP levels you probably should beef up the chassis by inspecting/rewelding all the factory welds, boxing in the "C" channel section of the frame and adding reinforcement like F41 suspension braces and aftermarket braces like between the rear shock towers, at the lower control arm mounts and tying the rear frame horns together. There is tons of great information on here. Post up if you need help finding it. You will be shocked at how flexible the factory frame is once you get it out from under the car.
