will they?

Is a 57 chevy rearend the right width to use on a 54 chevy? are the brakes interchangeable on them?

 

It can be...it'll fit under most any Chevy, but '55 was the first year for an "open" driveline...'54 are closed......and no, the brakes are not interchangable! '55/'56'57 are.
R-

 

i've got a newer 3-speed tranny to switch to open driveshaft. just wasn't sure about the spring perches and brakes working.

 

Spring perches should work too...don't know if the '54 has a "centering" pin hole like '55's & up do...doesn't make any difference tho, your u-bolts will hold it in place. Are you using the stock leaf springs or going to '57? Use the '57 brakes. You shouldn't have any problem connecting the brake lines and use the stock master.
R-

 

The 54's spring center bolt locates the rear axle and it's too for forward for the 55-57 rear axle. Most guys simply redrill the hole in the 55-57 axle's spring pad or use a lowering block and drill the axle locating hole offset to work in a 54 and down chevy.