whats my Horsepower gain with these mods?

No 225 that was equipped with a cat ever made close to 145 HP. The earlier engines were rated at 145 "gross" and that was optimistic.

If you are putting that Holley on a 4 bbl intake with a 2 bbl plate you might gain a little. If you are putting it on a factory 2 bbl intake with an adapter, don't expect much. They make an adapter to bolt a Holley 2bbl to a 1 bbl intake, which you should not use.

If you still have the early pellet style cat, removing it will probably result in some gain. Replacing it with a modern honeycomb cat will give the same result. The 1 7/8" exhaust system is the main obstacle to flow, adding a Flowmaster and 3" tip will just make more noise. A complete 2 1/4" system from manifold to bumper would help more, and the bigger carb will add little if anything without this upgrade.

Bypassing the A/C will only help if you completely remove it. You'll be removing 50# from the car. So, the improvement will be 1/4 to 1/3 as much as the car slows down when you have a passenger.

A .040 shave off the cylinder head to bump compression will help as much or more than the other stuff, and would be worthwhile. All of these together might get you to a "real" 145 hp assuming the rest of the engine is in really good condition.