The 904 can handle anything you’re going to throw at it. Now, the 7 1/4” and 8 3/4” use the same size brakes, 1 3/4” wide shoes and a 10” diameter. The line set for the axle itself would be different, but you may not need that anyway. The drums will be the same, and all the hardware. The backing plates would swap over to an 8 3/4” from your axle, but a 7 1/4” backing plate is held on with 4 studs, the 8 3/4” has 5. You would just need to drill the 5th hole. All A body 8 3/4” axles came with the small bolt pattern, so unless the axles have been changed to an aftermarket axle with the large bolt pattern, you can use your existing rims. The driveshaft would have to be shortened with either axle I mentioned in my previous post.
If you just want to use the 7 1/4”, there are literally hundreds of them available that were removed for a differential upgrade, and they would be cheap, or most times free. You can surely find one that is in good condition that you could put right in place, but the fact remains that they are undersized for any spirited driving.