Do you already have the ultrabell , they aren't cheap.

Sounds like your BB transmissions are cores , ignore the insane prices you see listed by the parts scalpers in the parts section here , BB CORE transmissions aren't worth that much unless you have some obscure number trans for a Hemi or 6pk application and even then ....

If you don't have the ultrabell yet it will be cheaper to get a new driveshaft with 7290 joints than to use a driveshaft that may only have 7260 joints in it now . Plus you have a spare trans , you aren't going to stick a stock 904 that was behind a 318 behind a big block and have it live for very long unless you drive like an old lady .