Okay, so it's winter and other than us slobs in florida -you guys can't boat. But you can think about them.....here is what I have been thinking about: Add blowers in July for birthday.

Goal - more midrange and torque up to 4800 rpm, better cruise speeds and some cool blingage. I dont care about WOT....really since it is a cruiser.

Existing: Twin 502s (one counter-rotating) with Holley 850 carbs, arneson drives, BG 1.5:1 transmissions. 3/4-inch fuel lines from pickup to mechanical fuel pumps. Motors are 425 very mild torque-optimizing CAM, 4 bolt, rollers, raw water cooled with gill exhausts.

Plans: (comments welcome!):
Pair of Weiland 177s with over-rev 111% and 2.11:1 drive ratio for 4.3 lbs of boost or pair of B&M 250s with 1.6:1. One would be revving opposite direction and driven off internal "lag" gear. Which makes more sense for low end and midrange punch? 177 or 250? $6,000 for the pair.

Upgrade mechanical fuel pump to 130 GPH pumps. Already have 1-inch hose from 3/4-inch pickup in tanks all the way to fuel pumps so I think Im ok on hoses. $250 for pumps?

Upgrade fuel filter - using racors now but they might not flow enough.

Send heads to machine shop for upgrades to exhaust valves - to iconel $2,000? Or is this overkill for 4 lbs of boost?

Get set of 850 CFM carbs pre-set up for blowers - I want to run single carb per motor still. Keep my existing Holley 850s for when I sell the boat or motors.

What do you folks think? Ideas?

I've had 177s on a 454. They were pretty much maxed out on that engine. Go with the 250s. I've worked on a pair of 250SC/502s with a single dominator, a single 850 will be too small. Idle wasn't great but ran well. Consider a pair of carbs rather than the single.
Check, you may be able to change the blower for reverse rotation by simply flipping the drive pulley nose assembly.
Head work wouldn't hurt, but you might get more bang for the buck with cam and good exhaust.
Upgrade the ignition with a V6 module, 30degrees max advance. Try an MSD if yoou have idle problems.
Depending on what you've got now, consider a bigger oil cooler.
Btw, what kind of trannys are you running?
My $0.02.

I have borg warner transmissions - 1.52:1 reduction - they are the ones with no down angle - straight - and they spin the same as the motors - BW71/72? Arneson ASD8 for the outdrives.

Are the Gill exhausts not considerred "good" exhausts?

Curious about the trans question

Thanks for inputs!

Oops, missed the gill exhausts. They'll be fine.
Since you're running a reverse rotation engine and a tranny with a reduction on an arneson boat, I assumed you were running inline ASDs.
I was wondering if this setup require the engines to be pretty far forward, and required driveshafts.
Also, how much hp is the 1.52 gear reduction tail capable of handling.

Huber told me about 500 max.

yes - I have double u-joints and a shaft about 3 feet long. The motors are five feet forward of the transom.

500 is what I was told by the bam guys last year - good call Rik!

good inputs guys - any idea what the torque curve will look like with 4-5 lbs of boost ?

250 blowers and single nickerson 1150's worked great on a 555ci motor out west, so for your 454s that set up would go nice they can put the whole package together for you and they are known as the carb guru's when it cimes to blowers. They did a nice job on a 1050dom for me this fall.

ah but will the 502 pistons survive?

any gurus in s Florida you can recommend?