What is the reason your swapping everything over to the fl60. Assuming fl70 is a bigger truck, I don't know? Is it for the cab only
Best way i can explain it..
Big is not always better
The analogy would be 1/4", 3/8" and 1/2" ratchets..
1/4" being 3500 and smaller
3/8 being medium duty class 4-6
1/2" being heavy duty class 7/8 trucks
Now any one that has turned wrenched knows the whole set overlaps.. while most have suggested get a full blown semi.. that's like hauling a 1/2" ratchet all the time for everything else.. can it be done sure, is it efficient... Yes and no 1/2" on a 1/4" stuff is actually slower than a 1/4" on 1/4" stuff.. due to moving the heft of the 1/2 ratchet all the time..
I have been running with a beefed up 1/4" ratchet, and the FL60 I'm building is basically a beefed up 3/8" ratchet.
The 8.3 is basically one of smallest engines one can get in a class 7 truck.. now a day smallest is. 10-11L engine with 300-350hp all the way to full tilt 600hp 15L
The FL60 is a class 6 truck. Im putting class 7/8 engine trans in it.
So why not class 7/8 axles as well? They are substantually bigger, the fl70 has a RS-23(23k) axle which has a 19" ring gear vs the FL60 has a RS-15 (15k)with a 13" ring gear..
Bigger components take more HP to turn.. simple physics
I'm setting truck up to be able to reliably drag up to a 30-40,000lb trailer.
While I could gain mpg with a smaller transmission The RTLO 13 gives me more options for cruising speeds and engine RPM pending load weight wind drag and terrian. which will allow me to keep the engine in the sweet spot vs choosing between screaming it or lugging the **** out of it.. neither really help mpg