Is there any chance the distributor is 180 out?
I've done that myself, did more or less the same thing you describe (I didn't seal the carb with my hand but it didn't backfire till I 'put both feet on the dash and pulled the choke ALL the way out' as my dad would say
).
Good luck, I know exactly how frustrating it is when a project doesn't want to fire when it 'should'.
