I'm in the process of restoring the stock drivetrain on my '51 and started with the rear end. It's now completely rebuilt with a 3.55. I have the SM420 4-speed transmision and I noticed the old 216 was a bit happier at speed but she's tired and will be coming out to be replaced with a rebuilt 235 so I can't be of much help on high speed impressions. One suggestion that I will offer is to look into some large-diameter LT tires. The ones on my truck now are only 26" in diameter and aren't helping the high-speed cause.
Driving around town, the change is dramatic and I think a lot of guys are overlooking the drivability of the SM420/3.55 combination. Essentially you have a fully syncro 3-speed and you never go wanting for a gear to downshift into. Dump a load of mulch in the bed and pull off in first gear slick as can be. The "granny" first is a lot more practical now... no more instant red line but still stump pulling power.
A lot of bad mouthing goes on about the stock drivetrain in these trucks but I, for one, just happen to think the 3.55 gear and the rock solid SM420 make a very drivable combination. I'll be glad when I get my 235 in so I can hit the 55mph roads. I'm not interested in expressway cruising at all. Where I live, it's mostly back roads anyway.
