I'm going to be pulling the factory 5.7 (315,000mi) from my 98 Silverado and dropping a crate motor in soon. I'm trying to put together a parts list ...
...What are some of of the parts that I should definitely replace vs swap over?
Updated fuel injectors vs. factory "spider" system
Fresh motor mounts IF YOU CAN FIND GOOD ONES. Good luck, all I saw was thin-steel junk, and polyurethane inserts that didn't fit properly.
Catalytic converter
O2 sensors. They'll never be easier to put the torch to than when the engine is out.
INSPECT THE DISTRIBUTOR GEAR, you can expect to need a fresh one. Possibly the entire distributor. The plastic Vortec distributor is a high-failure item.
INSPECT distributor cap, rotor, plug wires, plugs. Make sure the plug wires are routed properly as shown in the service manual.
INSPECT the flexplate. They're known for cracking.
INSPECT the torsional damper. Maybe the core engine comes with a new one.
Consider a replacement starter motor with heat-shield. Same for the alternator.
Flush the PS fluid, then install a PS filter
Inspect the PCV valve and hoses, air filter element, evaporative fuel canister, fuel filter
When you have the radiator out, be sure the AC condenser and any other fluid-to-air coolers are clean and have straight fins.
If you have an AIR pump on that engine, verify that the pump is good, the plumbing is leak-free, and the diverter valve works.
Depending on your location, install a core-plug engine heater
20 year old factory sensors are better than new OReilly's ****, so if it isn't throwing a code swap it over.
The key here is to connect a scan tool BEFORE you disable the original engine. LOOK at the data stream. You'll know if any of the sensors or actuators are screwy. The only sensors I'd replace "just because" are the O2 sensors.
Take a peek inside the radiator. You'll know right away if it's worth blowing money on. If the inside is clean then just blow all the crap out of the fins and stick it back in.
There's more to radiator problems than scale in the water tubes. I've seen plenty that have the air fins rotted away from salty road splash. Even if the water tubes are in good condition, if the air fins don't make good contact, the heat transfer won't happen and the engine runs hot. Then there's leaks at the tank-to-core seal.
Much as I hate to suggest a new radiator--because I despise Communist Crap, and pretty-much all radiators are Chinese junk--20 years on a radiator is getting to end-of-service.