You can use your existing mounts no probs there. I have said this numerous times you CANNOT use any other sump other than a 7k one. What details do you need to modify the sump? Trial fit the motor and see where it wont clear (midway drivers sideish) mark this area out, Then cut this out and get someone who can weld to weld in a modified (pretty much inverted) section. If the motor you a buying doesn't have an inlet manifold then there is no carby and a 3k carby will be inadequate for a 7k. So you will either have to go aftermarket manifold (eg redline) to adapt a weber or the sort or track an original 7k-c manifold and carby down.
Hope this helps.
Matt.