[38Chevy454]

Given that you do not have much actual money invested, you need to evaluate your time to build vs selling this truck and upgrading to a finished or bigger truck. I totally understand the thought of doing this and having your business, so you have the skills and ability along with the tools to do the job. I do hot rods as hobby and would rather work on them than build the T/C. I guess depends on your time and finances.

Concur with the suggestion that a bigger truck is going to be better. Also the lack of OD will hurt highway cruising speed and mpg. I tried starting with a class 6 medium duty Super C, and then upgraded to full class 8 conversion. Could have saved time and been ahead going with the bigger T/C first. Would not go back to medium duty chassis.

A real engine brake is nice, exhaust brake is not as good. Not sure if your DT466 has one? Bigger engine allows for better speeds up hills and then safer down with an engine brake (or exhaust brake as better than nothing). You will not have much mpg difference with bigger engine, since majority of the time the engine is making same power for a given situation. Having reserve power is nice.

Nothing wrong with manual trans, but do agree it can potentially hurt resale. The big truck "automatic" trans are really just computer controlled geared manuals - the computer does the shifting. You may find a big Allison and torque converter, but more than likely it is a real geared trans that just has computer controlled shifting and has a std dry clutch. The early autoshift had clutch pedal for starting and stopped, then not used once moving. Later more full autoshift do not even have clutch pedal in the cab. Kind of like driving a conventional automatic trans, but slower shifts.