Possible Quadrajet issue....surging at WOT...vacuum leak?

[fordguy40]

Distributor, wires, plugs, and fuel pump are new. Timing set to 16* initial, distributor provides 20* mechanical, vac. advance disconnected to eliminate a variable. Goodwrench 350 with around 5k miles on it. Carb has less than 100 miles on complete rebuild by me. Starts, idles, and runs at part throttle perfectly. Just started driving it a little harder and it has started bucking and surging under load, such as merging onto the freeway and holding the throttle open to maintain acceleration. When I left off the gas, it returns to normal. No backfiring, no smoke, no stalling.

Noticed a few things about the carburetor that maybe you guys can help me with. Really beginning to think I've got a serious vacuum leak, even though the propane test showed nothing. First, the airhorn bolts were a bit loose and the idle came down a little as I torqued them. Going to pull the carb and tighten up everything that comes up through the bottom.

Second, even though the throttle shafts have been bushed, there was noticeable play in the primary shaft when I took it apart. Is there any play allowable here?

Also, can't get it to idle below 750 rpm, even with the curb idle screw backed out all the way. The lever that the screw rides against also does not touch the forward stop on the carburetor body. Need to check the throttle butterfly alignment, but assuming they're not jammed, should the throttle shaft lever touch the carb body with the curb idle screw backed out and should I be able to kill the engine with the screw backed out all the way?