I have to buy a longer cable and housing for my 700R4 converson into my 64 Chevelle about 71" long.I may try a junk yard.

Are all GM cable connectors the same. Any suggestions as to what donor truck or car I should go for?

Yours is a screw-in cable into the speedometer. Later years are a snap-in cable. Think they changed around 69. Think trucks changed around the same time.