On the other hand

Uhhh.....
$1000 to rebuild a transmission for a slant six?

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You can BUY a good used 318 and 904 for less than that. Holeeee crap!
Are there no self serve junkyards near you? The 84-89 Diplomat-Gran Fury-Fifth Avenue cars had 318s, 999/904s with the low gear 1st and 2nd and can be pulled from junkyard cars for under $500 total. You would be way ahead by considering this rather than what you're doing.