Glenn,
Car had about 140,000 miles in 1993 when I was still driving it daily to work. I bought a truck and decided to park the Trans Am to repair the rear main oil seal and put headers and dual exhaust on it to make it more fun. Since I had it in the garage, that began my work in the motor inspecting the top end (I kept fouling one plug) and found a collapsed hydraulic lifter and worn down cam lobe. You will find receipts for work done by Reeves Performance (heads and carb rebuild) and engine parts ordered. I did the camshaft and bearings, piston rings, crank bearings. Of course Reeves did a full rebuild on the heads (springs, seals and valves) and I put new rockers, lifters and pushrods in. Water pump, oil pump, alternator, fuel pump, radiator, and all associated belts and hoses were replaced. I couldn't repair air conditioning myself and wasn't going to pay to fix it since I drove it mostly windows down and often topless. Also at some point the front suspension rod ends and ball joints were replaced (see receipt). Also pulled stock engine driven fan and installed electric fan.
I pulled seats and replaced the carpet, laying down sound and heat insulating mats under the carpet. There were no rust holes in the floor and any surface rust was cleaned and painted. By the way the car came new with some kind of undercoating nose to tail and anti-rust treatment. I'm sure the dealer did that to make more money but it also made this a survivor car!
Car drove well until 2003 at 180,000 miles when I heard a loud knocking. I was 3 miles from home so I drove it and parked it. I did a compression test and it was good. No oil leaks or even smoke from exhaust. It sat due to many priorities and I never did anything more than put oil in cylinders and button it up. You said something about distributer could be 180 out. My money says it's a wore out bearing on the crank pushrod or a broken rocker arm or lifter. Trust me, everything was running smooth and powerful up until the knocking developed. That was at least 40K miles of pure joy!
Things I've learned over the life of the car:
Stock tires are 225/70 R15 (kids who think they are experts will argue forever that those cars did not have 15 inch wheels). The tires on it now were put on just to move the car around. They are not correct size.
I've run 235/60 and 245/60 and at one point had 255/60 r15 and they looked the best and still cleared the fenderwells.
The GM ignition module in the distributor is known to go bad from heat. I've replaced it 5 times over the life of the car. When they start going bad, the engine misses and you get a check engine light. When they fail, the engine cranks over but will not start. About a $20 part and worth having a spare on hand. Pull distributor cap and two screws replace the module. Using the heat sink bonding paste is critical on that module.
In the trunk is new t-top weatherstripping (expensive), the centercaps for the wheels, 2 oil filters, and a new grill insert. I swear I had two but can't figure out where the other is so I think I must have put one on the car already. I also gave you my collection of OEM and aftermarket service manuals.
Where I would start:
New battery and have starter rebuilt (franklin battery does great work) and the rebuilt starter will be guaranteed to fit with existing shims. Larger aftermarket high torque starters won't clear the headers. Also needs an insulation blanket wrap on the starter to protect it from header heat. The last few years I drove it, the car would turn over slow after it got warm from heat soaking next to the headers. I probably killed the starter. If it's not the starter, it may be the heavy cables from battery to starter.
The fuel line is disconnected from the fuel pump on the front of the motor (near the power steering pump) because I drained old gas out of the tank 6 months ago.
I didn't want to pump old gas into the carb when turning the motor over.
Once the engine is turning over, a mechanic may be better able to diagnose the knock.
Good luck and I'd love to see it when you're done!
-Mike