Pontiac 400 that is about to make me pull my hair out.

[Pt109]

I see its been quite awhile and hope you have it figured out by now. I had an extremely aggravating issue with a 460 ford in my 4x4 years ago. Hesitation was bad, 3 new carbs, backfired thru carb, hence the replacementswouldnt run right, idle worth a damn and tons of other things. I also had the vacuum lines all screwed up. Had some of the best mechanics look at it and not one of them could figure it out. Long story short the diaphram in the brake power booster had a pin hole in it. Replaced that and it never ran so good. It sure did some strange things before we got that figured out. Just thought I'd put in in for another thing to check. The little things cause the biggest problems. Good luck.

[MonzaRacer]

Pontiacs with low compression is flat tops open chamber cheads can handle as much as 60 deg total timing. Closed chamber heads will not work with low duration cams ask me how I know!
Also older high Comp heads may take shorter push rod, also you should measure max amount of lift the springs,retainers and guide boss can handle, many Pontiac heads need help to handle higher lift cams.
On my 400 I ran my base timing at highest notch on cover like 14-16 I think, still have it may need to look. As long as it cranks OK hot and doesn't surge at idle its OK I like to use stock HEI with stock weights and lightest Mr Gasket springsxcfron curve kit. Get everything running well there, then hook vac advance to manifold vacuum then retune idlespeed and mixture.
Oh and Pontiac lifter will work in sbc but sbc lifters will drop out of the lifter bore, tip sideways and break lifter bores, the weakest point of Pontiac block. Hopefully you don't have older pre 65 block, different animal and doesn't handle later parts well.
Many stock casting heads won't take over 500 lift with certain retainers.
Oh and if you have headers dump the gaskets and go find a caulk type tube of dap/Dow Corning 50 yr clear silicone, slathers it well and install grade8 bolts with lock washers.
And if cam wasn't broke in properly you may have flat cam! When installing flat tappets cams use a moly based cam lube use high zinc oil like Brad Penn, Comp cams over counter truck oils to use are either She'll rotella t or Chevron Delo.