No way on the swap to a 474, keep the 5.9. Hop it up, just a little(free mods). With 250hp, it'll do really well. I'd say, up to about 40k pounds. Now, the trans is probably going to be the turd of an AT545, and 250hp is the max you want to send thru it.
turn up diesel fuel rate site of mine
67 F100, 300, 4spd, Offy 'C', Edelbrock 500, 274*/.518" cam, heddder.
98 Jetta VR6
82 Mercedes 300SD, fuel screw adjusted.