I have a 91 chevy c1500 truck wit 350 engine. I'm want to put lt1 engine in it. I have the complete wiring harness from under the hood and dash from the car that the lt1 came from. Can I use those har

U will need the ecm as well for ur lt1 along with the engine harness

On same truck... Will I need to cut under the hood to make a complete fit or up grade motor mounts?????

The lt1 is a 5.7 l engine the same as a 350 the motor mounts should be the same

So it should mount up and fit

Yes, just to be on the safe side if u r getting the engine from a salvage yard I would have them save the motor mounts from the vehicle that ur lt1 is coming out of all in all it really does not matter though the mounts are all the same on the small block Chevy's from the 4.3 v6 to the 6.0 v8

I think last question.... I only need the harness from under the hood and not the inside harness

Correct u need the engine harness, now in some cases this could be the entire harness but for most domestic cars and trucks u have two secondary harnesses that make up the main harness so with ur truck u will need the engine harness