I was out of town and missed this post 2 weeks ago.
I was just digging to see what was back here.
If you don't get answers in 5 days or so, just add a post to your thread and get it back to the top of the list.
The first thing that I would do is to disable the vacuum secondaries on each carburetor. stop them from opening, as a test. Just put a piece of wire on the linkage to hold them closed. It will not hurt anything.
They might be opening irregularly causing the surge problem.
If this helps you might need stonger springs in the vacuum cans to delay the opening.
The lean backfire could be caused by the vacuum secondaries popping open, since the secondaries do not have accelerator pumps.
Also I would reduce the timing to 28* for a while until things get worked out.
Blower ratio is the amount that the blower turns compared to the engine crankshaft. 1/1 is ok. faster is overdrive, slower is underdrive. The best ratio to be determined by testing and trial and error involving induction temperatures, boost pressure, etc. A larger lower pulley will make it over-drive.
You won't hurt the bottom end with 6 psi. You might run into detonation due to insufficient octane for the boost amount before the bottom end is hurt.
Just keep asking, and somebody out here will try to help you.