It's funny how big an impact a little information can have.
I finally pulled the rear diff cover off to figure out for sure what ratio was in there.
The original ratio was 4.68, I figured the current gearset couldn't be lower than 4.10 given the tire size and the current top speed.
I was wrong.
3.73 is what's in it now, this led to the calculator coming out.
Supposedly there is a 3.23 ratio available but the lowest I've seen listed is 3.31.
Things being as they are now this would increase the top speed by 8mph to 75 mph.
But when factoring in the 600 rpm I'd give up with the 8.3 i'm back down into the 60's.
This is unacceptable and based on my calculations, without an overdrive transmission I'm fucked.
Allison doesn't make an overdrive transmission that isn't electronicly controlled to my knowledge.
As of right now the engine swap is on hold while I brainstorm a solution.
If nothing else happens I will install a 3.54 gearset and run it with the 5.9
this will lower my rpm at 65 by 5 or 6% and shouldn't negatively effect towing.
I don't think she could pull 3.31 gears with the camper in tow.
On a positive note she gets 13.5 mpg when not towing, not bad for a 12,000+ lb vehicle.
That's it for now.