most all of what everybody said is good advice GASSER 1955 about the gov. sticking, the vacuum to the modulator, the modulator and valve being free, but on the 400 there is on passing gear linkage. its and electric switch in the trans that works off of a switch on the carb. or under the dash on some models. but if it goes bad it wont shift into any gear most of the time. will it back-up ok and is low gear ok? hope this helps.