Not to de-rail the OP's question but I would like to know if the 8.3 QSC engine fits in a 35 Duffy without a step up or engine box?
Pushing the motor ahead of the bulkhead is to achieve more deck space for work gear. Also when the motor is put that far forward larger motors fit. The big issue on all of these repowers is getting the transmission to clear the wineglass of the keel. The problem with putting the motor that far forward is a loss of speed do to lack of bow lift. The key of these types of vessels in addition to the correct gearing is placing the motors a little more aft.