Motor stumbling, input please!

['70 C/10 SS500]

I got the exhaust done on my '76 500/Th-400 swap into my '70 Chevy truck this weekend. I finally got to run the motor for more than a few seconds now. The motor was pulled from a running '76 CDV, cleaned up on the OUTSIDE, and only a new non-roller steel timing set for the inside. A new water pump, oil pump, plugs, wires, cap, rotor, 425 pulleys, and re-built stock Quadrajet carb top off the other mods to the motor to get her to where she's at currently. The motor has just under 110,000 miles on her. I set the timing in neutral (no vacuum adv.) to 8 deg. at a yet unknown idle rpm. No tach in the truck and no tools/gauges to measure rpm. If the shop that re-built the carb didn't mess with the idle speed then it's stock from the running car before. In neutral, the motor revs fast and sounds strong. No hesitation or stumbling at all. With the new exhaust she sounds wicked! (Stock manifolds into 2 1/2" head pipes to dual Flowmaster Delta Flow 40 series mufflers). In gear though, I can only throttle to about 1/3 to 1/2 throttle with good manners, but anything past that and the motor stumbles and bogs and doesn't recover until I let up. The truck was originally equipped with an inline 6, 250 cid/Th-350 and HAS (currently also) 5/16" fuel line from the fuel tank. I transitioned to 3/8" just before the fuel pump inlet and use the stock pump outlet to carb hard line to supply the carb. When the shop re-built the carb, they left the jetting all stock. The vacuum advance is pulled off the carb nipple directly above the fuel inlet (the 45 deg. downward angled nipple,) and goes directly into the vac. adv. pot without any tees to anything else. The carb only has two nipples used, that -45 deg. nipple to the vac. adv., and the tranny modulator valve in the back. The power brakes is plugged, and all other nipples capped. The carb shop says that the ported vacuum is the 1/4" nipple front and center of the carb at the base. But the stock Caddy vac. adv. line came off the -45 deg. nipple, so that's what I used. The egr valve is blocked off, and the vacuum manifold under the dist. on the front of the motor has all 5? nipples capped. The passenger side exhaust manifold heat riser buttlerfly has been removed and the hole for the buttlerfly shaft is welded to (effectively be) plugged. The weather outside when I was trying to tune her was California 60+ deg. and sunny. What could the stumbling problem be from? Anybody have any ideas? I am still in the begining phases of tuning since I just started this last Sunday, but it is stumping me at the moment. Please help me get this big Caddy past part throttle!!! I hope this makes sense and will help you help me. Thanks in advance!