The external dimensions of virtually all small block V8's built before the center-bolted valve covers came into use are identical. There's even bolt holes for the 4-point motor mount setup that would have been original to your truck. If you have a V8 in there now, a 350 ought to be a bolt-in swap, using the same mounts, bellhousing, etc. as the 283. If you choose to use the HEI distributor, massaging the firewall a bit with a big dead blow mallet might be required, but maybe not. Good luck!
Jerry
