When I did the TH400 conversion on my '64 GTO I had to use a '69 GP tranny dipstick and tube in order to clear the firewall. Probably the same as any '68 -'72 A-body TH400 dipstick and tube, but since all the auto '69 -'72 GPs had the TH400 I would look there first.
The B-body dipstick tube you have now with the long-tail TH400 won't fit.
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My wife says I never listen to her, at least that's what I think she said.