I have a 253 holden motor to put into a boat?

i have bought plans from america for a 18 foot hand made boat that weighs 745 pounds.so guess 325 kilo's.the motor i am talking about has been bored out 15 thou.the bloke says it wood be 263 horse power .he also says i need to change the crank and the crankshaft.to a cost of maybe 600?so all you out there let me know please