Timing question 318 with LD4B

Bought a 1987 B350 van to make for a camper for my boys. It has 318 engine that was originally propane powered, but was converted to gas by previous owner. It has LD4B intake and Edelbbrock 1406 600 CFM carb. I am getting hard cold starts and backfiring through carb. Plugs were new, new fuel filter and carb was cleaned. So I am thinking timing. I do not think anything was done to the internals of the engine, just intake and carb swap. What should timing be on this set up? TIA