I didn't want the OP to think he was being ignored, so I just gave him some info about different gears and tire diameter. Certainly a world of difference between his truck and mine but I figured some info is better than no answer.
Roger that!! Thanks!
Jerry --- You're probably right. I may have been thinking the 4400 axle gearing ... Or heck, even the 359 Pete's ... All I know is that in pure stock configuration, my 3804 could get to 55 mph, but it took most of the morning to do so and any kind of a headwind, bump or the slightest uphill grade would slow it on back to 45. Making it tedious in traffic and absolutely not capable of highway or long distance travel.
The simple upgrade of the rear axle 3rd member (4.11 gear set) was still too tall for the 216. Adding the 261 took Charlie a lot of places he'd never been before, mainly off the trailer and into the left lane

-- From Trailer Queen to Road Warrior!!