The SM420 was standard in 2 ton trucks and an option in pickups. It has a very low first and reverse gear. It has no purpose other than starting heavy loads such as a trailer. Your 54, if it has the components it was born with, will have a 248 engine with 3.90 rears with a closed drive shaft. I am assuming you have a 1/2 ton 100 series. My favorite way of getting more speed out of these things is using an open drive shaft rear out of a 55-59 which is also 3.90 and then installing a 3 speed with a BW overdrive. This gives you a .70 overdrive. 3.90 is a good match for the OD. Anything faster would make OD only good for going down hill. Cletis has a Suburban with a 248, 3speed OD and 3.90 gears and he put over 10k miles on it last year. However, I think he likes the smell of rubber burning and it may have a 302 in it one of these days. The Muncie 4 speed is a closer ratio 4 speed that was used in muscle cars. However, 4th gear is direct and you don't go any faster than what you have now.
