1970 C20 dilemma

[GMC69]

So here is my problem. 1970 C20 with a 396 big block and recently put a built 700r4 behind it. Runs good, might need a rear main seal soon.

A few weeks ago I picked up a 2001 4.8l vortec with a reman 4l60e, plus wiring harness and PCM, some 6.0 manifolds and a transmission cooler for 300$.

Engine supposed to have 208,000 kms, so about 130,000 miles
transmission has maybe 3,000 kms, so about 1,800 miles.

I opened up the motor cause I don't the history, even though the guy I bought it from said it was from his grandfathers truck and was going to install it in his 1950 chev pickup, then decided to go diesel.

The heads look nice and clean inside and cylinders still have some crosshatch left. Checked a couple main bearings and rod bearings and they look new. Hard to believe there is that much mileage on it.

The plan is to replace the oil pump, re-gasket the whole thing, new timing chain and possibly gears, new balancer, replace the valve springs with LS6 springs in case I decide to swap the cam later on. Clean up the exterior, long tube headers and a tune.
With the info I can find, I should be able to get it to about 330hp at the crank, compared to my daily driver 2014 Silverado which is 355hp. Not to mention my 1970 weighs about 1,500lbs less than my 2014 which is about 70ish horsepower feel due to weight difference.

So do I drop the LS/4l60e combo in my truck or keep the 396/700r4 setup and install a stand alone efi system. Pretty sure my 396 is one of the low HP versions. Just looking up casting numbers now to confirm, plus I don't know what was done to this engine before I bought the truck.