You can find a 518 trans dirt cheap(people dont know
they are a 727 yet... you might want to swap the input
and the pump to run a high stall conv(old style input
shaft)... the 904 is a fine trans and with a hopped
up 383 can handle it... but the 518 is better...you
will still need the bell change to fit a BB OR just
use a 727... the 518 offers a OD if you drive it on
the street... this is what I went with on my Rampage
and being a total new build I could have used anything
EDIT
You have to find a 1 wire OR a 3 wire type 518 trans..
these are the ones that work without a computer