Here's a little background on the engine.

It's a 1967 chevy impala 283 that is now in a 1950 chevy truck. It has been completely rebuilt. From the best of my knowledge it was bored 40 over and I put an edelbrock rpm intake and I believe a 650 edelbrock carb on the engine as well as an HEI distributer.(Could be a little off on the carb size, after all it's been 15 years). Also it has roller rockers on it plus new push rods, although only 7 out of 8 are new b/c at the time I lost one. I also put the rubuilt heads, intake and carbon myself.
The truck started out as a father son project back in 1995 and very shortly afterwards turned into just my (sons) project and unfortunatlely with school and work it kept being put on the back burner. Now that I have had a litle time and money I have started to work on the engine again and here's my issue, mind you the engine has been ran off and on here and there and even put under load throughout the last 15 years in order to pull the truck in and out of garages and storage units.
Ok so the engine ran pretty dam awesome before it was rebuilt and after we rebuilt it, it never ran the same and up until this point I felt all it needed was for the valves, carb and timing to be adjusted. For the last 3 weeks I have been attempting to adjust the valves and carb using the advice of a neighbor who used to work for edelbrock and it seemed to start up pretty well but it would backfire through the carb a lil bit and would putter out and die as soon as it was put into gear or accelerated whatsoever under load. Ok not a big deal thinking it needs more adjustment but today I started it up and out of nowhere it backfired louder than a shotgun out of the exhaust and sputtered out and died even faster than before under load but what worries me most is that out of the right side exhaust (it has dual exhaust no y pipe) there is white billowing exhaust fumes. Any help or advice would be greatly appreciated. I just want to make this thing run decent so that I can be excited again about working on it. As I sit here the truck is stuck in the drive way unable to move under it's own power and the rain is coming again, mind you I was able to drive it slowly up and down the street the other day without to much of an issue until I pushed to much on the accelerater and it would sputter out and die.

Thanks again for the help..