Sounds like you need to advance your initial timing. As the rpm picks up the timing advances and the backfire goes right. I can't even get my 351C 4V to idle half ass with less tha 12 deg initial advance. I use a mechanical advance only, easier to curve. Also, depending on your cam and carb you could be richer at idle than at say constant 3000 rpm because the vacuum could be low enough to open a power valve (if you have a Holley for instance). Try bumping th timing up just a little, you should be able to get a decent idle with no backfires at around 800 rpm.
69 Sportsroof, 351C, 4spd toploader, Restomod BOSS 429 clone - the one and only (?) 1969 BOSS 351
http://home.swbell.net/boss351/mustang/boss1.jpg
http://stangnet.cardomain.com/user/adamg69
