63 Dodge Dart newcomer questions/advice

Cool car I love the color. Looks like a solid project.
First off welcome aboard glad to have you!
As far as your question I can't help much I dont know alot about the early cars but there are alot of members who do and I'm sure they will chime in and help. But I do know know this if you decide to change the transmission get one that came off of another slant six the slant 6 to v8 transmissions are definitely different the bellhousing bolt pattern is different.
I scraped a perfectly good slant six transmission not long ago I wish I would have kept it now.