To clarify a few things, if you have flattops they will give you very close to the same compression as the factory rating for the heads you are using. In the case of the #48s, that would be 10.5:1. If you have dished pistons, the volume of the dish will be added into the total volume to tell you your "new" compression ratio. As stated, no compression numbers without piston and deck height numbers.

Aluminum heads will allow you to run a higher compression ratio on the otherwise same engine combo and the same octane gas as with iron heads. That extra is .4-.5, not 1 point or higher. A more modern chamber design on an aluminum heads will allow higher than that but the rest of the engine needs to be considered before counting on being able to run pump gas.

The extra 1 1/2 points of compression will add only about 8-10% to the power output or about 30 hp, not 75-100hp. A fairly stock 400 with 8:1 compression and a mild cam will make about 300hp, a better cam about 320-330hp, a 400 built strictly to '69 RAIII specs will make about 360hp, add properly ported heads for another 30-40 hp, and finally another step up in flow to the Edelbrock heads for about 450hp. Proper cam selection and engine prep is assumed (but not always easy to find).