Intermittant starting line bog

Had my car(74 Dart 440/727/3.91 w/spool gutted interior sub-frame connectors 8 pt. roll cage) out twice at two differant tracks with the new motor.(Specs below) First pass off the trailer I've run 12.98 and 13.0 at 105 mph both runs. The car pulls hard through the lights at about 5600 rpm. After the first pass it will bog off the line and sometimes die completely. I am launching at about 1200 rpm because my 235/60R15 drag radials don't hook real well. I'm running 12* initial and 32* total all in at 2800rpm. I was planning on bumping the initial up once I got the new motor broke and everything else sorted out. When we went to load the car up it fired up and died and backfired through the carb. You guessed it, fire in the hole. Got the fire out. Pulled the carb at home expecting to find carnage, but it looked clean inside. Wondering if I should rebuild this old Holley or saving up for a new carb. I have a 3310-2 on my street car I was thinking about putting on and resetting the timing. Elevation at Medford is about 1,400 and I think Redding is a bit lower. The car has always seemed rich at idle. Can't get much adjustment out of the two mixture screws. Thanks for any replies.

'73 440 .030 over 9.5-1 pistons
Stock 213 heads with hardened seats and Comp springs to match cam
XE274 Comp cam
Intake .488 lift 230 duration @.050
Exhaust .491 lift 236 duration @ .050
Holley blue pump regulator set at 7lbs.
Weiand single plane intake
Stock electronic dist.
MSD 6A box
Accel coil
About 3,000 stall w/ reverse manual vb
Carb: Holley 4779 dp
Primaries 76 jets white acc. pump cam can't find number on power valve
Secondaries 78 jets pink acc. pump no power valve not even threads to put one in
#32 pump squiters on both sides. I believe it has the large acc. pumps.