Swap question

Sorry all for lack of info,it originally had a slant 6 with 3 speed,I imagine rear was small but not sure,do the motor mount kits to swap a V8 or do i need to change k member? Also what size headers for 3inch exhaust? Also would love to have 15 inch wheels and large rear good for street and strip ?is 3:91 rear good for that? Also what year are interchangeable with mine from other years and models ty so much
It is a 1967 Plymouth valiant 2 door, previous slant 6 3speed ,