Just some thoughts:
The 4.2L V6 in that truck is more powerful than the 4.9L inline 6 in the older trucks, yet the older trucks (I have owned two 4.9L trucks, dad still has 1) has PLENTY of power to move around, and the 4.9L is very close in output to an RC1 BHW which is still a 2.0L 4cyl.... so
in theory an ALH could be made to be at least as powerful with the proper mods, and the truck will already be geared pretty good. Durability may suffer, BUT you could certainly do some things to help that along. Lower the CR and run a giant turbo with higher boost, plenty of room for a mammoth-sized charge air cooler, a huge extra oil cooler, cooling system could easily be made to keep things under control there... lots of room under an F-truck's hood.
My T3 (Vanagon) has a higher GVWR than my 1993 2WD F150, yet manages this with a 70hp 4cyl. Curb weight is much lower of course, and I know it would not ever win any races. My [new-to-me] T4 is almost as heavy, and does OK with a 109hp 2.5L gasser. Again, no races will be won, but I just drove it non-stop 75 MPH home from Vegas, and it never whimpered a bit and still somehow managed 20 MPGs
.
The Mazda trans in the F-truck has many versions, with many bellhousing setups. Someone else here is putting an ALH in a Ranger, with the same basic trans (just a diff bellhousing pattern) so maybe look to see what he is doing. Maybe sourcing a Ranger version M5R2 trans would work, you could use the same adapter.
Finally, I for one don't really feel that the 4.2L V6 bolted to the manual trans is exactly "gutless", and it already is capable of getting into the 20s MPG-wise with little load. You may not actually be gaining much in terms of a 4cyl TDI swap in the dollars/sense department.