[lowrollin70gmc]

I would take a good look at the new used motor and see if there is evidence of a leak. I don't like pulling anything apart that doesn't show signs of leakage. Often times a new gasket can also leak and no need to fix what's broken. With that said, the oil pan and rear main are done in minutes with the engine already out.

The 4.3s have a reputation for consuming intake gaskets so it's worth while to replace them. I like to use the upgraded Fel-Pro gasket set, as it includes steel based gaskets as well as valve cover gaskets, throttle body, etc.

I recently swapped engines in my 97 and both motors had new intake gaskets, but I had parts in hand so I installed the nice FelPros. I was going to do the rear main because it's so easy but it had a fresh one installed when I pulled the flexplate. I kept the stock senders as they aren't too bad to install in the truck, and a new distributor cap/rotor and wires never hurts.