Really need to run the numbers. compare the total drive ratio in first with the current rear, then do the first gear ratio with the proposed tranny and lower rear and then you can see exactly what you will get. For bumper to bumper traffic it could be good. just let it idle in first and creep along.

I would expect first to be non syncro, but the rest should be syncronized. I do not know this for fact, but it is the normal way a truck tranny is done.

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1977 240D turbo