With approx 226 deg [email protected]
in A 396 THE 4.0 P/Valve can setup a too lean of a siutation for shure but the 76 prim is plenty,may actually be too much when you get the correct p/valve installed.
I would expect that cam to have approx 12"-13" + intake vac at idle in neutral and lower in gear so with that said you should use a 6.0-6.5 p/valve.
With the 4.0 p/valve you have to basically be allmost WOT at over 3/4 throttle or @ WOT to get it to open with the cam you have so it will be running lean when getting into the throttle hard. Thats because the 4.0 p/valve wont start to open like it should at that time in a moderate throttle position that's less then 3/4 throttle to richen things up even though the motor is under a fairly big load at like close to or at 3/4 throttle neediing more fuel then the main/primary jets alone can provide so try a 6.0 or 6.5 p/valve.
BTW,a 6.5 p/valve is stock for most holley perf carbs not the 4.0 you mentioned ,that's for a motor with a lrg cam that idles with approx 6"-8" of vacuum.
Keep the timing at 18 base and your 38 total with the vac adv hooked up to full intake vacuum all the time.
I bet that will fix the problem or at least get rid of most of the caugh/stumble along with a smidge larger prim accel pump squirter but replace the p/valve first because i know the 4.0 is no correct for your cam & motor.
Remeber you may have to back off the prim jets from the 76 it currently at to maybe 72-73 after getting proper 6.5 p/valve installed or it may be fat/rich. That's because the carb will now be giving the proper enrichment the motor needs with the 6.5 p/valve at mid throttle whereas prior to installing the 6.5 p/valve someone tried to cover up the stumble/ caughing carb extreem lean problem at mid range to WOT with more prim jet then whats needed when running the proper p/valve that enriches the primary fuel calibration when required so keep that in mind.