I am not an expert, but from what I have read, the 478 would be a difficult swap. The 478 was externally balanced vs. the other GMC V6's that were internally balanced. The 478's only came in the large trucks with the large truck transmission and flywheels. These transmissions and flywheels would not fit in a small truck.
The 351M and 401M would be the ideal swaps. The M designation is for magnum. These engines came with larger intake and exhaust ports. They would be a bolt in swap with your 351D. Check out the following website for the experts. They have an excellent discussion group.
http://6066gmcguy.org/ Regards,
Steve
