chrysler 3 speed manual dodge rv 360 Swap

This is in my VIN and it has a slant super Six. So I should be fine to mate either the 1975 Dodge RV LA 360 or the Magnum 360?

What about the flexplate?

It will work if you get the dual pattern V8 bellhousing. Since it's a slant transmission, it will have different first gear ratio if I remember right. There is no flex plate in a straight drive, those are only for automatics. You will need a 132 tooth flywheel balanced for whichever engine you decide to use.