It sounds like pulling the engine for a refresh is exactly what is needed. Until it's apart you won't know for sure how far you need to go with it. It's probably original with a standard bore. It probably has a ridge at the top of the cylinder and some taper so a overbore is needed. .030 over is a standard and oversized pistons shouldn't be too hard to find. Get the high compression domed pistons with 2 valve reliefs to keep it "Commando" Your machine shop should boil the block and install new cam bearings and core plugs. They should also check the deck for straightness and machine if necessary, clean and measure the crankshaft. It might only need a polish or possibly need ground. Usually .010/.010 cleans things up nicely. Those are forged cranks and are very tough and long lasting. Most shops check the crank bores for proper alignment but rarely do they need line bored. The stock connecting rods are plenty strong with full floating pins They can be resized if needed and it's recommended if new bolts are installed. Hone the lifter bores to make them happy. You want the new lifters to turn easily in the bores. If they don't turn, you can ruin a camshaft in a real hurry. Let's see, did I forget anything guys?
I used a Isky E4 cam and lifters. It is close to factory specs and still is a solid lifter cam like the original. The 273 is a small cube engine. Watch how big of a cam you install. Too big can get you in trouble
That manifold number is correct and original for your car.
Again, until the engine is taken apart and wear is noted and measured, you won't know what you need. I didn't use the Egge "kit" but I can see it would be handy because of all it includes.
Remove, disassemble, check and measure and have a plan of attack. Your machinist will guide you.
Let's dissect your original post.