What transmission is in my fathers truck? It's a 3-speed with overdrive manual. The engine is a I6-300(4.9l).

 

Probably an NP435 but more info is needed to be sure.

 

A 4 speed OD is not the NP435.

This is PROBABLY the top shifted version of the Tremec built, redesigned for OD transmission originally known as the Ford T&C "top loader" 4 speed from the good old days. Pics here, first ones up:

Domestic Truck Tranny Guide

Top shift and aluminum case would ID it as the third one they describe.

 

I think that's what it is, I know its got an aluminum case and the floor shifter goes directly into the top of the transmission.

 

Made by Mazda!!

 

The Mazda's were 5 speeds.

 

There were no 5spds in 1984 f 150s, the cast iron case outside linkage 3spd ODs were of American source the alum top loaded trans were made by Mazda. Don't recall when they began using the mazda trans, my 79 was cast iron my 84 is aluminum. Straight 5spds were not available until the redesign in 87.