Slowly but surely

[Docs fitty6]

For those who might be interested, I'd like to say that I have put over 50 miles on my '56 GMC shortbed stepside Rat.
I've been working on the bugs for a couple of weeks since.
The original radiator wouldn't hold the heat of the BBC 396. DUH
Installed a new Champion 3 row HD aluminum radiator and 16" cooling fan. Temps are great now.
O-ring at the thermostat went tits up and had to be replaced.
The o-ring at the governor cover at the trans sported a leak...it somehow got pinched at rebuild time. It has been replaced and does not leak anymore.
The exhaust is too loud and a Super Trapp diffusser baffle system was welded to the exhaust tips.
The carb had to be re-jetted up and the metering rods had to be downsized and the vacuum can at the dizzy had to be downsized to a 12" vacuum pull from the one that was there requiring over 18" of pull. The springs and weights were replaced based on the distributor machines readings. I don't know exactly what those changes were as they were performed by someone else at the Break in time.
The ignition switch does not turn the engine off unless it is turned to 'accessory'. I have gone through everything and am thinking it is the ignition switch itself and am going to ask for a replacement from Ron Francis Wire Works. (I love their kits!)
The front tires were 195-45R15's they sucked! But it's what I needed to roll the truck around until I got the fenderwell headers re-configured. Now I have some 205-65R15's on there.
I've had one experience with a hot start problem...but nothing since then.
My voltage regulator went tits up too. A new alternator has cured that problem.
So now, the next big thing I'm trying to fix is a high speed miss-stall-bog-misfire.
I'm not sure what might be causing it.
It starts fine, idles fine, revs great w/o a load on it.
36* total advance at 2500RPM's
750cfm Edelbrock main jets are .113 (were .110), metering rods are 70/42 (were 75/47)
NGK Iridium IX plugs gapped at .037" (Too small a gap???)
Lunati cam assymetrical pattern 276 int./284exh. lift at 1.7:1 is .554int./.572 exh.
Fuel press. set at 6PSI.
Little backpressure through exhaust.
Running premium fuel with a cheap octane booster (103+)
I'm going to go over plug wires, distributor mechanicals and timing to see if I can resolve this problem.
It doesn't have the high speed miss if I gradually pull the RPMs up..only happens when I stomp on it..
I have the HEI performance module...I had to replace the super coil back to stock due to fit/interference with the plug wires and firewall.
Any ideas from your experiences?
Got to title and license this thing so I can do more driving!
later

[Wheelie]

Sounds like it could be a float setting or a weak pump shot based on your description. Maybe you need to run even more timing and are you running vacuum advance? and if so whats your engine vacuum? Too little and your vacuum advance wont work. Also are you running your advance off ported or manifold vacuum? Another tip is I like to set my timing by advancing it until it runs on after shut off then back it up a few degrees until it stops. Basically advance it as far as I can.

Other things to consider is what compression ratio your engine has( just to make sure you dont have too much requiring race gas).

[Docs fitty6]

I've changed out the stock vac, can. for one that comes on at 8"HG.
I'll go through the accel. pump arm linkages and tighten everything up.
Running manifold vacuum (non-emission controlled engine)
10.5:1 C.R.
The plan is to go through the carb a bit more...pump cam adjustments, float levels and fuel pressures (currently @ 6PSI..may increase that to 6.5PSI) and I have yet to adjust idle mixture screws and I know that is critical too.
I have an idea that it is a 'Lean Stumble' as it wants to 'Die' when the throttle is stomped.
I have so few miles on this motor yet (less than 60) and don't want to do a bunch of WOT pulls out front of the neighbors, just yet, in order to see exactly what occurs during bit longer with my foot in it. I've hesitated pulling it hard during the initial onset of the hesitation/bog/stumble.
Thanks for your help. It has some of the ideas that I need to explore.
Doc

[Docs fitty6]

I've gone through the carb again just to verify some things re: high speed stumble. Found a couple of items. One float bowl out of spec., the accel pump arm in the wrong slot and the pump linkage not secured correctly. That's fixed.
Then the timing. It's 20*BTDC for the initial, 18* centrifugal and 13* vacuum for a total of 51*BTDC.
I hooked up an external tach and adjusted the idle mixture screws.
Things with the motor sound and feels good. So I took it out for a spin and it spins up to about 5200 R's under a load and then has small amount of miss/stumble yet, BUT, much better.
The plan now is to trade in my HEI for an MSD digital 6al, a performance coil (running a stocker now) and Mallory distributor with programmable mech. and vac. adv. curves. I especially like the MSD Soft Touch rev control.
I'm pretty sure my dizzy is just plain running out of steam much above 5K RPM,s.
I've got over a couple hundred miles on the motor now.
I'll be content putting a few hundred more miles on until my tax return comes so I can spend it fast on go fast goodies.
later