8 3/4 gear swap

Have a couple of questions. Never messed with swapping out a gear set up and will probably let a reputable shop do this. But was wondering about.

I have a spooled 4.56 set up right now 8.75 rear end. I am looking to swap them out for a for 3.91 or 3.73. My question can I reuse the spool from the 4.56 or have to buy complete set up?

I am have 295/50/15 tires on the rear. Which would be a better gear 3.91's or 3.73's? Installing a mildly built 383 BB.

I will be using car for mostly street driving with occasional trip down the 1320.

Any help would be greatly appreciated.