This may or may not be good news to you but I will give you a similar example and it could be your problem as well... What condition are your heads in? and your block and engine itself?... I was having a really bad stumble/hesitation and couldn't figure it out, i had just installed a new carb and assumed it was a vacuum leak...Come to find out, 3-4 days later my 289 threw a rod due to a bent valve at the top of the cylinder (which was also the cause of the seemingly apparent vacuum leak)...Your problem could just be a carb in need of adjustment, but i thoguht i'd put my .02 in just since i didn't see any other posts as this perhaps being a possiblity. good luck

 

ok, I said it before in my other response, pull the vacuum line off of the distributor advance and then crack the throttle little by little. mine did the exact same and I found that by accident. The rubber diaphram in the advance unit was hard and did not move and the best part of it all, it was a single port advance on a set-up that was for a dual port and the P/O had also removed the thermal vacuum switch, gotta love it. My car had sat for over 6 years and $300 later it runs like it is brand new!

 

OK - the idle mixture screws are adjusted properly- no change to the hesitation - still there. The carb is rebuilt - no worn throttle shafts or anything loose to cause a stumble condition. No manifold nor carb gasket leaks - indicated bythe normal (20-21 inches Hg) vacuum gage reading at idle. Per the previous post(s) the distributor vacuum advance is new - and holds pressure when vacuum applied with my Mity-Vac.
Also, due to the correct compression test results and the correct manifold vacuum at idle - and the fact the engine runs strong off-idle all the way up inRPM's - I'll assume the heads and block - in fact all of the related machined mating / sealing interface surfaces are OK - no warpage or leaks.
Given the relative good (based on recent testing)'pumping' ability of the engine as the basis - I'll concentrate on going back over the initial timing and carb settings combination, such as the accelerator pump and linkage.
Great suggestions - keep 'em coming!