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I was thinking of compensating for this with a different speed gear. Does this gear come out of the housing the same as on the ultramatic?
Similarly, but with different types of parts (P/Ns) between standard (Grs. 3.256-3.257) and Ultramatic (Grs. 3.681-3.686).
Unlike later GM vehicles that I've worked with in this regard on, the spread on speedo pinion (driven) gears listed for standard and OD in the Packard V8s is only a bit more than 10% (depending on which way you look at it), and there's only one choice of driving gear for overdrive (another for standard trans).
You should first determine the ratio of the current rear axle to see if you have the right tooth count on the existing speedo pinion gear. Without knowing the full history of the donor car, it could very well be that some other axle ratio had been installed that isn't regular production - not supported by any available speedo gears.
You might want to consider an inline speedometer ratio adapter. Though not exactly cheap, they may be easier to find and provide a more accurate reading than any available gears for the transmission.
Mind you, that's all assuming that your speedo head was accurate, to begin with.
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