I think you are on the right track. The car is looking good and I think at this point you could rebuild the brakes and drop in the 454 and redo the fuel system to ensure years of trouble-free weekends. It is always easier to spend other peoples' money so I can tell you exactly what you need to do! On a serious note, the LS engine is cool but it would probably end up being a nickle and dime deal just to get it in and running correctly. I guess on the other hand it all depends on if you will keep it fuel injected or slap a carb on it. Either way to be correctly done you'd probably be around $1,800 to $2,000. The big block on the other hand- it looks like you have a lot of the stuff to put it in. I'd pull the pan and inspect the insides, yank the intake and check that stuff too. If you find someone with a borescope you could put it into the exhaust ports and rotate the engine till the exhaust valves are open on the cylinders you are checking to get the camera in. You may even be able to slip it into the spark plug holes. If the cylinders check out, I'd pull the rod and main caps to check the bearings. If they look good, you could get plasti-gauge and check the clearances. If everything is within tolerances, slap a good Melling oil pump in it, bolt the pan on, prime the oil system and do a compression check and a leakdown check. I bet if it all checks out fine, you could have the big block bolted up under the hood for $500-$600 and have it all painted up underhood.
You really can't do anything with the suspension until the whole drivetrain is in it in order to align it correctly. Things like the battery tray and inner fenders and the front end fastener kit will probably run you another $300 shipped to your house for a rough estimate. Another plus side is you'll still have the LS engine in the garage for the RX7 if you ever feel the need to play around
