I'd agree to check the squirters first- then you need to check your timing. if it's good then you may want to re-curve the dist.
Rule of thumb is 2x the cubes on carb's.- I run a 750 cfm Holley on my 327, and it's a tire fryer. But I also have 11 1/2:1 compression pop-ups, and 63cc high flow heads.
Backfiring will cause the old Holley's to rupture the pumps, which is why I suggested checking the timing & the dist. curve to see where the backfire may have started from.
Good Luck & let us know when you get it fixed.