Welcome
We want to see pictures of the truck.
In the "project" area start a post on the work done and the plans of work to be done we all like to knoe what others are doing.
What 4sp transmission do you have?
I ask because there are 2 different types.
T18 / NP435 both have granny first gears and driven as 3sp starting in 2nd gear and only used the granny for heavy loads.
The other is a 3sp where 4th gear is a over drive.
SROD comes to mind.
My truck started with a T18 and I swapped in a NP435 with a over drive unit as the T18 was bad and did not want to lay the truck up when rebuilding the T18.
I also have a 300 six in it, love the combo of the six & granny first gear & over drive.
Dave ----