Steve, maybe I'm confused again. You know I get that way at times. I think neilnip wants to go faster and wonders if putting a SM420 from a 55.2 would make him go faster. High gear would be the same. He would have a first gear he would never use and a reverse that would terribly slow. Any of the later 4 speed overdrive transmissions would make him go faster, but they are usually only .80 overdrive. However, they came out of passenger cars, not 55 model trucks.
I also think that morethanaframe has a SM420 now and has a chance to buy what I would assume to be a SM318 with a Warner OD unit for $100 which is a bargain. I have seen them fetch $400 on eBay. Maybe I'm making too many assumptions. But the 3 on the tree linkage will probably work on the front section of the overdrive transmission. The only difference in a regular SM 318 and an overdrive is the mainshaft. The housing is the same. Even if he has to spend a couple of hundred on a relay, etc., it would be a cheaper way to go faster.
The main reason I like overdrives is because I have been fooling with them for almost 50 years and I used to show tail lights in an unmentionable make of car on this forum with an overdrive to lots of 283 and 327.
My reason for preferring OD because I like a 3.90 or 4.11 around town. And of course out on the highway, I can drop the rpm by 30% I have driven some 3.55 rears and they lacked the snap away from a red light that I like. I think a 270 or 302 in front of 3.55 is just fine. If I ever win the lottery, I'm gonna buy a bunch of Tremec boxes.
If' Ive gotten way off track, I'll go drink a beer and maybe think straighter.
