I have been following this thread. I read that you said you has 15 inches of vacuum before it backfired. You also said you replaced the old power valve with a 4.5 power valve. To get the right size power valve you take a vacuum reading at idle and divide it by 2 and replace it with the closest number to your reading. EX: 15*2= 7.5. Summit racing list 6.5 for the 600 cfm. While you had the carburetor off you should have backed off the curb idle screw. About .020 of the two slots in front of the primaries should be showing. Next you would need to setup the distributor If setting the initial timing at 20* you will need to set the mechanical advance at 16* max. Do a search on here about setting the mechanical on a HEI. This is with the vacuum hose off and plugged from the distributor. So you have 20 initial + 16* mechanical = 36 total. Now you ready to fire it up and make adjustments to the carburetor. Hook up a vacuum gauge to a manifold vacuum source and adjust idle screws until you get the highest reading. To check the total timing set the dial at 34 and check timing. it should be reading close to 0 on the scale if you have everything set right. You asked about the springs on the dizzy, yes you can use 2 light springs so the timing is all in by 3000 rpms.