360 Backfires Under Load

Howdy from JetCity. It has been nice weather and just in time, I finally got my car together and out of the garage. Now I am trying to get it to run properly- and failing.

I have a 1976(?) 360 stock rebuild from a local machine shop. I bought a Lunati Voodoo 256/262 kit, MP springs, Mopar electronic ignition and a K&N filter. I added a Holley Street Avenger to my Edelbrock TM5 and a 2 inch exhaust with Flowmaster 40(?)s and an H pipe.

I mentioned in my welcome thread that I am new at this. I guess I may add that I am having a hard time understanding how everything works together.

I have had the car for three years. I want it to drive around running errands and to take it on the freeway off and on. I would like to get a trans and rearend and go racing.

Before I spun a bearing the car seemed to work well on I-405 and around town. I had a leak on the primary side of the carb and could not keep it in time with the advance on the dizzy. It would surge. But with the 318 and Holley 1850 on a TM5 and dual point distributor, factory manifolds (273(?) cast iron) and a single exhaust it was fine and fun.

After deciding not to put any money into the 318 I went with the 360. After a few hassles I got it going only to find pretty good leaks at the manifold flanges and a severe backfire when I accelerate(anything more than slight pressure, usually). I drove it to the gas station and filled it up, I drove it to the muffler shop and I drove it to work and back, and a little test and tune tonite. I retarded the timing way back and checked for misfires. I did not change my intermediate shaft during the rebuild. I have done nothing to the carb(NIB Holley 570 Street Avenger) but put on my air filter and choke, set the idle.

What is up? :sign3:

What is going on?