I'm working on my '51 club coupe,swapping in a 318/904 that I had lying around,wondering if anyone has done a similar swap,and what rear end did you use? I've heard a jeep cherokee rear will fit with spring perches flipped,but which gear ratio would be best to look for behind the 904?

Under the sheet metal, the frame & suspension isn't much different then any 46-48 Mopar. There is a lot of info concerning this swap here and several other places, a Google search should find you hours of reading and pictures.
Basically, you off set the motor & trans about a inch and a half towards the passenger side so the exhaust clears the steering column and make motor mounts.
The Jeep rear end is a good choice, as well as several others.
According to my book, the shipping weight of a 51 Coronet club coupe is 3410 lbs, and the flathead 6 your removing weighs more then the 318/auto your installing. Pretty much any gear ratio will work out. Personally, I'd go for something in the 3:21 range, but a 2:76 would work as well, limited slip optional. If you use the OD trans, a 3:55 works well. Gene

check the D/24-P/15 SITE lot of those type cars over there good luck nice project