Proflite transmissions run modified 904 internals in a 904 case.

They typically run a modified AMC 727 case (the bell housing is smaller) with an adaptor plate. The adaptor plate also helps in the use of GM convertors (they're taller and more plentiful). To allow this the 727 pump is modified, a custom stator support and input shaft is used. Dave also relocates the front band's adjustment point in the case as well.

TCS also offers replacement parts nowadays. My understanding is that Pro Trans does all the machining for the A1 Proflites as well.

Protrans also do other styles of race transmissions as well.