Mini Cooper rpm

Changing the engine has nothing to do with the gear ratio. 3,000 rpm at 75 sounds normal to me.

Thing is before the engine swap it was less rpm and it felt good. Now you can even hear the engine inside the car and it sounds weird.

Unless your clutch is slipping the engine speed would be exactly the same as before.

I think it might be slipping then. Because when I got my engine switched the mechanic did tell me it was going to need changing in the next couple of months

Too bad you did not change the clutch when you changed engines.