I had a 86 460 like yours . The rpms almost followed speed . It screemed at 55MPH
The manaul for 87-90 had the following trans options .
C6 3 speed
4 speed manual
the manual 5 spd or the zf trans.( one you want)
And m5od Mazda trans . Most commom F150 trans
Zf is a solid trans. You will need transfer case to match if wont work with one you have . Drive shafts and crossmember. Might not need all but in case. The ZF also have hydraulic clutch. Which means you will need slave cylinder and stuff. Cab floor panel .
As for the auto route . The newer e4od is electrically controlled . But according to manual was to be had in your body style . As well As Aod trans but was always told it is a light duty trans. Some one will chime in as to what needs done electronically . But same BAsic parts may be need for it to . Crossmember,driveshafts ETC .
Goodluck