I wouldn't worry too much about being under powered in that boat unless it is important to the new owner to go 5- 10mph faster than a comparable boat. 454's for 30k less would be my choice. They should go for 1500 hrs maybe more but 40' is big. Look at all the 39 SRys with gas and they still sell well. I do agree diesel is the best but with that comes the extra cost (which will be re-cooped some at resale). Your not hurting anything at all with 454's except 'ye ego some. Kick back and enjoy the ride.