HELP with 350 stalling issue

[JFC10]

I have a 69 C10 longbed with the 350

I recently put a cam in it, specs are:
.420/442 lift
204/214 duration @ 50°

I also converted it to HEI and slapped a Edelbrock 1405 on it, with a spacer to convert it to work with the stock intake manifold.

Other than that the truck is stock.

I have a weird issue with the truck dying when you give it more than 3/4 throttle. Mash it from a stopp, dies, mash it while coasting, dies. I have my timing set at 4° advanced. Bumping it around does nothing to remedy the stalling.
At idle in park/nuetral, you can rev it up and there's some hesitation, but it doesn't die. I'm thinking the hesitation might've been a hint, so I messed with the accelerator pump adjustnent and that does nothing either.
What causes a aftermarket engine to stall like this?
What should the timing be for a slightly better than stock cam?
I'm out of ideas, I'm sure someone tuning wizard could hear it running and fix it in one step.