If your talking stock v8 compared to stock 6 the six is about 75 to 100lbs lighter, now if you start adding aluminum heads and intake manifolds and blocks, then things change but thats cheating :P.

Also my six was never rebuilt when i got it, head was just fine, engine was ran too hell, no fluids ever changed, PO had "tried" to resolder half the radiotor back in, carb had no float balls, and it was running on five cylinders due to a spit up lifter. That damn engine fired up every time, and after i rebuilt the carb, ran every time. Now my sisters 289, thats another story, running on all 8 cylinders, with people taking care of it, developed a tap shortly after we got it, and now is sitting in our yard with a frozen engine.

What six cylinder did the posters car have? If it had a 200 then i'm shocked that he lost the bottom end, as those are bulletproof. Now if he had the crappy crappy crappy 170 then i can understand, those things hate highway trips.

I'd personally say if you want it ready now, replace it with a six, if your cool with waiting and working on it, replace it with an 8. Realistically depending on how old your springs are, i'd say replace them with v8 springs, its not expensive, don't cheap out on things like that (you can get some decent ones new for less then $100, used for cheap also). Do you have disc brakes or stock six cyl brakes in it? If you have the stock six cylinder ones i'd reccomend upgrading too 5 lug disc, since after all if your getting five lug in the back it would be good to match it up, plus the fact that you'll want the better braking power.