Exhaust timing has some play in it but really need to know the blowdown, For that size engine .100-.105" is a good area to start. Some engines have to have a high exhaust port (180 degrees) to reach this number because of the transfers being higher. a ultra thrust is the same design as the "Jett" mufflers. They are both 1/4 wave pipes. what is the crank timing on that engine?