Unless you are going to upgrade most aspects of your Mustang (which will cost more than a few dollars), you should stick with the 235 hp engine. Its torque curve, compared to the other two engines, works just fine for your intended use. No way in heck should you even consider the 390 hp engine.
My 66', pictured below, has about 325 hp and has numerous upgrades to keep me and the car in one piece: Radiator; Monte Carlo bar; roll bar; built T-5 tranny & driveshaft; frame connectors; traction bars; 9" rear end w/axles; performance suspension/brakes; etc, etc, etc.
Even if you only upgrade to the 300 hp engine, you are definitely going to have to upgrade your car.
