what transmission

[stevework]

hello, i have a 1981 f350 in need of a replacement transmission.are the t18 and np435 both options?
the truck is a 351 windsor 2wd. the original transmission is long gone and the one being replaced is a np but it's height and width are different from the original.(although it did work for a year) i'd really like to be able to get the proper one this time, any advice would be appriciated

[HIO Silver]

A TKO-500 would be my choice...3.27 first gear and a 0.68 overdrive.