New to this site and hope to get my '63 Dart GT convertible back on the road after FORTY years. I have a very tired 170 in my '63 dart with 150K miles. I also have a rebuilt 225 with 2k miles that came out of a '71 Valiant. Can I swap the '63 170 for the '71 225? Will the '71 mount to the '63 transmission? What else would I need? Thanks for the help.

Welcome!
You need a bushing in the back of the crank to support the smaller nose of the torque converter.
The rear flange changed around 68.

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Hyper_pak

In addition to the adapter bushing you need to use a 68 or newer flexplate, the engine brackets and motor mounts from the old engine. oil pan and pickup tube from the old engine, and carb with linkage.
One thing about the 170 CID carb. If it is the original carb, it is smaller than the carb for the 225. It will fit and work, however. The reason to keep the old carb is due to the linkage hookup.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225