I know it isn't a 4x4, but I need some help. I have a 1969 Chevy C-20 with a 350. I am building a high performance 454 (500+ hp) for it and am looking for some polyurethane motor mounts for it. The biggest engine offered for this truck was a 396. I know the blocks are the same, but if I find mounts for a 396 (Prothane) will they hold up to the power?

If you really want to hold the engine down and use polys, try to find 1972 frame mounts. The 72 year model used the same mount as the 73+. Those were the 'clamshell' style and polys are readily available. Try this site for parts
http://67-72chevytrucks.com/vboard/