What you want is to "reseal" the engine. Before removing the engine from the vehicle, I'd verify the timing chain/gears (timing set). I'd also do a cranking compression test, and/or a leakdown test. No sense resealing an engine with a weak cylinder or two.
If this were me, I'd buy an overhaul gasket set, and replace EVERYTHING including the head gaskets and valve stem seals. Be sure the head gaskets you end up with are the same thickness--or a bit thinner--than the stock gaskets. 0.040-thick head gaskets are ultra-common, but I bet the ones on the engine are only 0.028-ish. When you have the rocker arms off to do the valve stem seals, inspect the pivot balls and the end of the rocker that holds the pushrod, and the other end that pushes on the valve tip. Not uncommon for the valve tip ends especially to be worn. Look for excess deposits on the valves. You "probably" don't need to have the valves/seats ground, or even lapped--but you'll know more when they're disassembled for inspection. With the heads off, you'll easily see whether or not the honing marks are still on the cylinder walls. That's not unusual, and it's very good. Years ago, you'd have a bigass wear-ridge at the top of the piston ring travel at ~200,000 miles. Not so much today. The Vortec short-block I rebuilt only had 0.0015 cylinder wear. Amazing, to me.
I'd also expect to open-up the oil pump for inspection; and--likely--sanding a thousandth or three off of the housing to tighten-up the end-clearance. Look at the screen on the oil pump pickup; if there's debris caught in it, you might want a new oil pump and pickup tube. Sometimes the screens can be cleaned.
Not a bad idea to pop off the front main-cap to check for main bearing wear. Similarly, you could yank the cap off of the #1 connecting rod to look for rod-bearing wear. Torque the bolts to spec when you're done. I wouldn't expect excess wear at that mileage, but as long as you're in there, you might as well look.
If this were me, I would (and did...) pull each lifter apart ONE AT A TIME so that all the goop can be cleaned-out. Hydraulic lifters are like tiny oil filters. The passage for oil getting into the lifter is relatively large. The clearance for oil getting out of the lifter is microscopic. Therefore any particles that are pumped into the lifter are trapped there until you disassemble and clean 'em out. DO NOT mix parts from one lifter to another. Ideally, you'd keep the lifters in order so they go back onto the same cam lobe they are on now. I don't think this is "as" important with roller lifters as it was with flat-tappets.
Make sure the gear on the bottom of the distributor is still good; and that the bushings in the distributor housing aren't wiped-out. The Vortec plastic distributors are a known wear item.
The flywheel should be inspected closely for worn teeth on the ring gear. Flexplates are known for cracking around the bolts that hold it to the crankshaft. Not uncommon for the "hub" to break free of the rest of the flexplate--but that won't apply to you.
Since the intake manifold is coming off anyway, this is the perfect time to replace the "spider" injection system with the upgraded electronic injectors.
You can inspect the engine mounts, but I don't know of any suitable replacements. I put my old ones back in, because I couldn't find new ones I was willing to use.
Make sure the torsional damper is still in usable condition. Common for the rubber to degrade. Remember that the key on the crank that aligns the damper, and the ID of the damper needs a thin film of RTV silicone to prevent oil seepage. Doesn't take much.
There's a hundred different brands and varieties of engine assembly lubes. Some are "thick oil", some are "thin grease". Pick one. It's not critical what brand you use, just that you use "something" as an assembly lube so nothing is put together "dry". Even plain ol' engine oil can be acceptable--depending on how long between assembly and engine start-up you're expecting. The thicker the lube, the longer you can delay start-up. Cylinder walls get a smear of engine oil or ATF. I use ATF. And VERY LITTLE.
If you're not experienced, the "worst" part of this whole operation will be getting the lifter preload adjusted properly. Ideally, you'd have a mentor on-site to help. The tricky part is getting to where there's no clearance in the valvetrain, but not tighter than "no clearance" (Zero-lash point.)