'66 327 in C20 issues

[pjb155]

I've been having serious issues with the 327 in my 66 C20 pickup.

all i know about the engine is that the block is also from 66 and the size matches the build sheet, so could well be matching.

At idle, it runs ok and revs reasonably but somewhat resisting to rev high. Under load i'm having constant backfiring and sometimes blowing back through the carb. Power is so limited the truck struggles to accelerate at all in 4th gear (1.00) and as i'm driving a bit further even struggles in lower gears, making it basically undriveable.

It used to run much better (or less bad), but an oil leak resulted in a short highway drive with no oil pressure with loud ticking noises as a result which went away after refilling with oil (20w50). I'm not sure if the issues started immediately after this "little" incident.

I ran some tests to see if I could identify the issue(s):

InsHg at idle 14" steady
BTDC timing 5° adv with vac advance plugged at idle
cranking compression around 80 psi on all cylinders variation +-10 psi
cylinder leakdown test indicates some leaking via the cylinder walls / piston rings into the crankcase, but is still in the "low leakage" range on my leakdown tester.
Fuel pressure 4 PSI at idle, 3 psi at high rev (stationary)

I know the compression is very low but i'm not sure if this is causing the symptoms or if i could avoid putting in a crate engine or going for a full rebuild...

Distributor cap & rotor, coil are all new. it has a 4bbl carb which has a triangular tag on it with T02512511 and E6 on it, i haven't been able to find anything on this, and therefore have not touched the carb apart from the mixture screws.

thanks for posting your views