Many racers are running a 904, if it's in good shape, you should be OK. Advantage in your case is you won't have to have a new driveshaft made, or get a new slip yoke.
Other than possibly converter balance, "it's a bolt in."
My recollection is you will have to "washer" one (two) bolts on the left side engine bracket. Not a biggie as I recall.
