If this (pic) is what you're looking for, it has to match the gear that's on the transmission mainshaft or it'll be eaten up when you start driving.
This page and the following have information on what speedometer drive gear and shaft housing (shown in the picture) are used on various vehicles. In order to get your speedometer close to correct, since you have a 3.55 rear axle gear set, you'd get closest with a speedometer gear set out of a passenger car transmission which have faster rear axle ratios than trucks (3.72 vs 4.11 for instance.) Either that or you'd need an adapter that goes on the speedometer driven gear between the transmission and speedo cable. With the info out of the parts book and some math you might be able to determine the speedometer drive and driven gear set that would work without a ratio adapter. But finding that combination would be difficult, as would be finding a ratio adapter.
Also be advised that the housing has a straight thread, and putting a pipe (tapered thread) plug in it won't work. I don't think the thread size matches any readily available bolt which might be used with a gasket, as the thread is fairly fine pitch, likely not a standard SAE fine thread.
