Another posi/"suregrip" question

Had good advice on my last question, now somthing a little deeper...deep for me anyways.

Point first...I would like to keep my 3:91

My rear has a 8.75 rear, case ID "741" with the 1 3/8 small pinion and 3:91 gears. My son has a tight "742" case with 1 3/4 large pinion with 3:23 gears. Although these pinions differ in size, can the 3:91 gear (ring) be installed in place of the 3:23? I understand "I" may have just answerd my own question, however I'm not sure if the pinion gear alone on the "pinion end" can be changed or what.....or will I have to get a 3:91 large pinion.

Any takers on this one. Thanks