Sorry I never got back to you about getting a small block into the 6400 series truck. Yes it can be done and the only real problem is the steering as was mentioned. I dismantled an '84 chevy C70 with a 366 and Eaton 5 speed. I kept as many parts as I thought I would need. I scavenged the motor and tranny mounts, the trans. and engine. The 366 is just TOO big to fit so I went with the 350. It bolted right up to the eaton trans(I was a bit worried about that). The steering box had to be moved outboard 1/2" to clear the exhaust. I cut the tube flush with the top of the box and cut the shaft 1" longer. I shortened the column from the '84 and used a double D intermediate shaft with u-joints on both ends that I scrounged from a junkyard. I ground the steering box shaft to a D style and put a CR brand seal p/n 7940 into the steering tube on the top of the box where I had cut it to seal the steering box. Works just fine. But now I have come into possesion of a set of small block manifolds that actually sit above the spark plugs instead of below. I am thinking of putting a stock column/wheel back in as these will clear.
Good luck
Dave