Is it a 1/2 ton or ???? An easy swap would be the 4bt 3.9L cummins 4 cylinder (a shortened 5.9L). Cheap and readily avaialble in old bread trucks and reasonable weight. I think it has or can easily be turned up to 150hp. It is also reported to have the same rear bellhoussing pattern as the sbc. The 5.9 isn't much longer than the 302 but the weight is almost double the original six (~1klbs). The popularity of the 5.9 also makes it rather spendy. You can just about buy a used 855 cummins for the same price. If this were a big truck you may want to consider the 8.3L cummins. The only reason to do a diesel swap is the novelty factor. You will almost never recoup the cost compared to gas. I would also be concerned about the brakes if it is a heavy trailer.

Jason