Eigth digit of the VIN will tell what engine it was.
K = L05 which is a gutless TBI 350
E = L03 which is an even more gutless TBI 305 (I know E is for the passenger cars, but not 100% sure for the trucks)
Z = 4.3 TBI engine which loves to suck fuel and go nowhere while doing so.
I highly doubt it was a diesel or a 454. Of course I am ASSuming you have a 1500 2WD. The NV2500 was never put behind eithr of these engines as far as I know. The NV4500 would have been behind either of those two engines, and it is pretty damn distinct. Cast iron case 5 speed with a removable bellhousing.
All of the above are damn near impossible to kill though. Lol!
I don't know who told you all 2WD trucks were 4.3s, but they don't know what the fuck they are talking about. The 454SS was a2WD truck for fucks sake.
Bottom line, if it is a 1500, and it does NOT have a removable bellhousing it is almost certainly a NV3500.