318 Tuning Tips - Help!

Hi guys.

I'll get this out before I go too far - I'm a panelbeater, not a mechanic, so please don't blind me with science. This is my setup -

Low compression 318,
Stock cam, dizzy (single points) and heads
New HT leads and plugs
Edelbrock 4BBL intake
Holley 600 (Vac Sec) manual choke
Headers with 2.5" dual exhaust - 1 muffler per pipe
904 Auto - standard

My problem - the damn thing is backfiring back through the carby. I have set the timing by ear to the point where it only just pre-ignites under real heavy load. A friend has adjusted the float levels in the Holley for me. The car is going pretty well now, but it has a real cough and backfire through the carb when I stand on its tail, before standing up and f**king off. Does anyone know what the cough and backfire means? :scratch:

Any help would be really appreciated.

Cheers!