grand national rearend swap

i'm in the midst of changing the rearend in my 1956 pontiac its getting up there in hp and the my factory 8.2 doesnt seem to wanna take the abuse..i baught what was left of a grand national mainly for the rearend for $150 and i was told there reallly strong... its a posi and 3:42 gears i think and i was wondering if anyone has run them in the tri fives and how they work and fit...i've herd about people using them in a 1955 chevy which runs the same frame as my car but i was curious of how they fit and if they are shorter or longer than stock... any help would be great