GEOO

I had an AQ311 in my 18 Donzi too...She ran 58-60 mph.
I purchased a new prop shaft from Eddie Marine shop. The prop shaft had merc type splines so I could run all merc props. The next year I installed a 383ci small block, the boat ran 66-68 mph. That fall, I switched the lower unit to an old race lower unit called an Erive. With the same engine I picked up 8-10 mph and ran 76-78 mph. The E:drive lower units have not been made since the eary 80s and you can't get parts. The next year I installed an Arneson and she ran 80.
And haven't looked back...

The old Volvo 270,280,290's are great strong drives, but they do not work well over 55mph. They have way too much drag. A Bravo would be 3-4 mph faster.
An Alpha would just break, don't do it.
There are two ways to switch from a Volvo to a Merc.
1) Glass in the transom and cut a new hole.
2) Purchase a Merc adapter. The adapter bolts up to the Volvo outdrive holes in the transom and has new smaller holes in the adapter for you to bolt up the Merc drive.

toolz

C Spray- Thanks for the info- that's what I needed to know. Working from your figures, I should end up in the mid to upper 70's, about where I want to be. I will be testing the E4's next week, and if they aren't enough, I may be interested in your worked ones. I was tuning and breaking in my new motor this morning using the C4's- what a joke! Made my 27 Fountain act like a ski boat. Thanks again- Toolz