Is it a HEI or points distributor ?
More of a carb problem than timing I would say.
A few things would help, plugs and a good set of plug wires.
I never had any luck with accell as they get shot very fast.
I have had better luck with mallory.
Is this a mechanical or vacuum carb ?
On the backfire I presume its thru the carb, that's caused by a lean spot. Plus not knowing if it has a choke on it, if not it will have to get on the warm side before you can drive it as the choke makes it more rich and stop's the backfire.
IM running two 650s on a cross ram intake and have to get to 100 degrees before I can drive it without the lean backfire.
I removed the choke housings on them, plus some other work.You can see them on my home page.
Instead of writing a very long guide on tuning a Holley type of carb here is their site, it has a free trouble shooters guide and manual that can be downloaded.
http://www.holley.com/HiOctn/TechSer...o/FMCTech.html
Berry Grant has a site also, but is the same carb type as Holley. To get started with put the screws at 3/4 turns out.
After you adjust the floats then you should use a vacuum gage to adjust the screws on the metering plates.
On the lower side where the throttle is, a screw for the idle.
This is all described on Holleys site.
After driving it for awhile it would be good to read the plugs so you can change the heat range if needed.
If you need any other info please give me a PM or respond to your posting.