what the difference between them and will a v8 one bolt on a a i6 and visa versa

what models are you talking about??

5ltr efi starters are on their own and the e series 6 cyl ones will not fit.
the BA 5.4 starters are made by Jag and are unique to the BA.
the small EB to BA 6cyl starters will fit the early V8,s if you cut a small section on the nose of for clearance.