8.3 Cummins vs. DT466 vs. CAT 3116 in an MDT

Not an 18 wheeler but bigger than a car question...

I'm looking at MDT trucks and these seem to be the three most common offerings in the trucks I'm looking at. I'd like to find a FL70 Freightliner box truck to use for my conversion. I want something stouter to pull our farm trailers with. We tote around 24' GN Stock (14k loaded), 30' GN Flatbed (24k fully loaded) and in the future a 24' enclosed bumper pull (10k - 14k max). I want to take a box truck, pull the box off, shorten the wheelbase and put a 10' - 12' flatbed on it with gooseneck hitch under the deck and pntle hitch and 2" receiver on the back. I would use the box to convert our flatbed trailer frame into the 24' enclosed bumper pull. There's the back story.

Here's the question.

Which motor is going to be better for what I'm wanting to do?
CAT 3116
DT466
8.3 Cummins

Which transmission will be better?
6 sp. manual
Allison auto

My goal is a reliable truck that can pull my trailers around and do general farm duty. It's down to basically a FL70 (8.3), IH 4900 (DT466) or Chevy Kodiak (3116). I'd like to stick to the mechanical motors since the truck will spend more time sitting than driving and be simpler to work on so all the trucks would be in the early 90's era. I need to keep cruising speed at 65 mph safely (2200 rpms or less) and mileage in the 8 - 12 range which shouldn't be hard considering our M2's loaded with 28' boxes still average 8.5 m/g.

What combination of tire size, gear ratio and transmission (final gear) ratio be to achieve my goal of 65 mph cruising while maintaining enough get up and go to get moving?

Any other thoughts or comments I'd love to hear them.

 

Bry- What's your dad got behind that 8.3? 10 sp? What's the hp spec on that one? His trailer looks about the size of what I'd like to be able to pull.

BAYOU- What was the weight of your FL70 with nothing on it full of fuel? With the 3.70 gear what was your cruising speed? Do you think it would have made a big difference had it been equipped with say a 4.30 or 4.80 gear? How long was the bed of your truck? How close to the rear of the bed did you mount the GN ball? I'm concerned about the tailswing of the bed hitting the front of my trailer but don't have any similarly setup trucks around me to compare and measure. Even with the 3126 not being strong, where do you think it sat compared to the 8.3 as far as power and reliability? One of the trucks I drive at work is an M2 with the little C7 and it's really doggy but being a lease truck, they've got it turned down so far it's hard to tell what's motor weakness and what's company preserving their truck. Of course having 450,000 miles on it doesn't help the power situation either.

Most all of the trailering I do is on "flat"ish ground. I've never had call to take my personal trucks over the Rockies (yet) or the Appalachians (only once or twice but lightly loaded). To put it in perspective, the trailers I have "can" be hauled by my 1986 F-250 with a 6.9 but the lack of braking power (on the trucks part) gets scary going any place that even has a small set of hills or even worse heavy traffic. Oh yeah, not to mention the horrendous lack of power. But hey, for the early pickup diesels I can't complain. Motor still going strong but too many heavy trailers caused the C6 to give up the ghost.

My other thought process behind the Allison was for the wife. She has some shoulder problems and shifting that six might get old for her on a long trip. Anyone have any experiences with the early to mid 90's Allisons? How much of a beating would I take on fuel economy? Any particular ones to stay away from? Any particularly good ones I should look for? Last one I drove was an old Ford F900 with an Allison 4 sp. I wasn't impressed but it was an old truck and I'm hoping they've made some improvements since then. The truck was stout and the gear reduction action of the torque converter helped it get off the line smoothly but it seemed like it had to rev to redline before it would shift and the last 3-4 shift the whole thing just fell on it's face. Pretty sure it was behind a 3208.

Thanks for all the input so far guys.