If go with a 76t or 77t spur gear like recommend that will also hurt my top end.
Are you SURE that a 76 or 77 spur really IS what Castle is recommending?
First thing I'd do would be to find out if Castle is talking 64 pitch or 48 pitch gears... In a TC4, with 48 pitch gears, an 18 tooth pinion with "the biggest spur you can get" is going to be a LOT different than with 64 pitch gears.
If Castle is thinking 64 pitch with their recommendation, you will NOT be able to get a big enough 48 pitch spur in the car to avoid cooking your motor.