We did quite a few similar projects in the mid 80's . My first questions concern what is backing the motor up and how much of a gain are you expecting to see. The 305 marine motors were identical to their over the road bretheren, low compression cast pistons and crank. They will respond to the normal upgrades.

Q1 - What's behind the motor? If it's a small I/O then check to see the torque ratings. We tore the bearing casings out of a couple of Volvo-Penta outdrives after stepping on the 305's.

Q2 - Is it an enclosed engine compartment or open? The motors like better flowing exhausts and intakes, do you have room for both? With riser exhausts starting around $800, be sure you can fit in a free flowing exhaust system before you go too deep into the motor.

Q3 - How much of an improvement are you looking for - 10/20/30% or more? The switch to a 350 gives you 15% right from the start, the parts costs after the core charge are the same.

Regards, Mark