Chevy 350 Dilemma

[UntrustedAlly]

I bought my 1968 GMC C2500 with a non-running Chevy 350 swapped in. The previous owner provided little information on the engine and truck, so I have figured out what I could and have done some work to the engine myself.
Engine/Truck Specs:

• ~1976 truck block, intake, heads - all stock as far as I know
• Holley 570 Street Avenger, rebuilt
• Aftermarket cam installed by previous owner, audibly bigger than stock (unknown specs)
• New Street Fire (MSD owned brand) distributor
• New plugs and wires. I don't think I've run the truck enough for the plugs to have showed fowling as I have not noticed raw fuel or buildup of any kind
• Aftermarket headers, appear to be Hooker or similar
• Engine vacuum is erratic, jumping between 8" and 16", possibly too bad to run the advance
• Compression test: 70psi across the board with the choke and throttle mistakenly completely closed - no blow-by, oil usage, or smoking, however
• Rocker press-in studs are bad: some are stripped, one is stretched. None of the rocker nuts screw in without an unreasonable amount of force.
• Rockers are in good condition, pushrods straight
• New fuel pump, intake gaskets, carb baseplate gasket
• Timing chain and sprockets gone through
• All vacuum ports plugged except for the vacuum advance
• SM465, Dana 60, 4:10 gears - 60k miles

Additional Details:
The truck starts, idles, and revs without issue. In gear 1/2 throttle+ or high RPM makes the engine stumble and sputter but I have driven up to 60mph being light on the throttle. The truck was originally a V6 so I believe in addition to the heads needing work the fuel lines might be too small. Truck constantly lunges when idling in 1st gear, no issue when throttle is applied. Cylinder #3 which somehow got 0psi compression though the cylinder's plugs are fine and the headers get just as hot as the other cylinders. I'm assuming user error on that.
I believe new or reworked heads would solve the majority of issues.

Dilemma:
My goal with this truck is to make a solid 350-400HP on 91 octane or E85. Possibly more HP in the future. I'm aiming for a fun street rod. Budget wise, would I be better off fixing and building up my current 350 or should I consider an LS swap? The engine obviously needs a lot of work but I have access to machine shops and I am willing to put in some man-hours. Budget accounting for ECU's, transmission adapters, high pressure fuel pump, etc.