What size tires do you have on it? Your effective gear ratio is about 3.55 by those numbers, so in guessing 205/70/14 ???
I would guess the 400 came out of a car with a 2.73 rear and has a 39 tooth speedo gear in it, you have to much error to get rid of it with a speedo gear change, you would need an adapter as it sits.
Changing to a 2.69 rear will get it in the ballpark, close enough to get rid of the rest of the error with a speedo gear change. best guess is you will have to go from a 39 tooth to a 41 tooth and you will have to change the housing to the one for the 40+ tooth gears.
As far as speedo driven gears, housings, and the o ring goes, you best source is your local GM dealer. They still carry all the parts...
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65 Bonneville Brougham 4DHT
65 Grand Prix 2DHT......now parts
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