ugly rear end

I have a 7.25 rear end and am switching to an 8 3/4. I have been told that for some reason since my car is a 1973 that i have the small bolt pattern on the rear axels. I was wondering if this was true and if i got a rear end with the big bolt pattern how big of a pain it would be to drill out new wholes on the axel...or if that is safe?