69 c10 questions

[billy37380]

i have a new rebuilt 283 sprint motor in my 69 c10 it has less than 2800 miles on the rebuild.

when i am sitting at a stop lite you can hear it kinda backfiring is what it sounds like it is not real loud. But when I am driving down the road it runs fine and drives great except when i coming to a stop from 45mph to a lite you can hear it a little bit then. Also I noticed last nite that there is blue smoke coming from the exhaust when I was sitting at the lite but again when driving it doesnt show any smoke