possible timing issue with my 87 K5 blazer

Hi everyone
I am new to this forum and appreciate the willingness of people to engage in these activities. The problem - I have a 1987 K5 Blazer which I put a new motor in and am having what I think is a timing problem but not sure. It starts and idles fine but when I put it in gear it runs very rough and is not drivable. Sometimes when at idle I can get on the throttle and it backfires terribly. When I reassembled everything I put everything back in place (or at least think I did) and then set the timing according to the placard on the front of the truck. Engine at top dead center, dist point to #1 piston. Started the truck and it ran pretty good. Then I disconnected the cable to disable the comp set the timing as directed and plugged the cable back in. Truck starts right up and runs good at idle but not with a load. Any ideas would be welcome and appreciated.