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I'm in the works of building a high HP mud runner ... I was able to get my hands a 1991 350 4-bolt main out of a 1-ton 2WD ... will the Engine be able to be dropped in a 1995 3/4 ton 4X4 with out and problems ?

*NOTE* both trucks do not have A/C

Also

The original 350 oil pan was replaced with the 4X4 style oil pan.

Any faqs, hints, or tips will be awesome !!!  

Is the 1995 truck a Vortec? If the fuel injection systems are the same, you should have no problem. If the fuel injection isn't the same, I'd swap over all the parts from the '95 engine to the '91.

The great thing about GM smallblocks is they all fit the same. I'm sure you can just bolt the older engine in the newer truck, but making it run could be a little more complicated.

What transmission are you running?  

I agree with Phil - you're likely going to need to grab the ECM for a '91 motor and do quite a bit of work.  Depending on its age, your '95 may have OBD-II computers, and the '91 would - if anything - have OBD-I.  

It's probably possible to just swap the injection parts from the newer block to the older one. That would give you a lot better performance and it's easier than trying to mix and match wiring and ECUs.

To change injection, you'll likely need to swap over the intake (and possibly exhaust) manifolds, and other fuel delivery components. It isn't difficult, but it will require a little more time than just dropping the new engine in the truck. I think the results are well worth the effort. I'm doing a similar swap on one of my trucks; it's going from carb to EFI and there are a lot of little things to do, but it's all factory parts so I think it will be reliable.