that 26 bertram is an incredible boat, one Of my custoemrs has one with twin 4.3 MPI's, it runs close to 60, The express is my first choice of an offshore boat. Cummins Mercruisers' r&d shop is here in Charleston, they just repowerd a 26 with twin 150 HP diesels, Capt said it runs great and sips fuel. The best set up I've seen on one of those boats was a guy in a Yamaha class one year, he had a 26 express, he converted it to a center console with a small tower, put a bracket on it and ran a single 225 Opti, boat would run high 40's before he painted the bottom(lost 4 mph), still cruises at 30 thru just about anything. I called BS on his speed, but after researching it, on this web site
http://ccridlin.home.mindspring.com/
they had an original boat house bulliten from Mercury where one of the express' was tested with twin 150(135 at the prop) outboards, got close to 60 mph, you figure in that those motors weighed in over 350 each for combined out put of 270 hp, figure the single opti 225 makes close to 240 hp and weighs 400, it starts to make sense. I don't think the flybridge will perform this well, but its something to think about. If i were going to do it, first choice would be Volvo d3 with the composite drive, d4's with the dpr drive, but either one would be expensive. Probably the most economical way to go would be a pair of 140(or 175) fourstroke Zukis on a bracket. Those older Mercruisers can be a money pit if they stay in the water( its what I make a living on)