For your project of fitting a jackshaft from a 1995-1997 Vmax 800 into your 1994 Vmax 750, you're right in noting that modifications to the inner chaincase are necessary. Ensure you get the correct part, as there may be slight variations in different model years. You can either source a chaincase from a 1995-1997 Vmax 800 or modify your existing 1994 Vmax 750 chaincase. The 800's bearing and seal will be required.