Classic/Antique Car Repair: 1972 chevelle, compression stroke, cam gear

Question
hi i have a 1972 chevelle that me and my father are rebuilding we took a 350 small block from a 84 GMC and rebuilt it. boring it over and doing other small upgrades in the process.we have put a fourbarrel holley carb on and are having problems geting it to start. all it seems to want to do is backfire. the timing is defennitly correct but we just cant seem to figure it out if you could help out in some way that would be great thanks alot

Answer
When you say the timing is definitely correct, how is this verified?  Did you personal align the cam gear and crank?

How are you sure it is not 180 out? Did you check the valves on rotation for the compression stroke?

Have you preloaded the valves or what position are they in?