Which transmission should i use?

Romeo said
Other than the A500 and the A518 where there any other mopar OD transmissions that would fit a 75 Valiant with a low mod 318?

Either of those "fit" unless your doing major tranny tunnel surgery. Your best bet to get an o/d without cutting the tunnel would be to use either a gear vendors unit or a GM 200r4 (which is smaller then a 727 or 904.