What length K-plane trim tabs should be used on a 34' V-hull. Currently boat has 17" cheap Dana tabs. Boat has big power so looking to go with K-planes.

That's right on the edge between K280 and K380, according to Merc's table. You'd be fine with either. Hydrodynamically, they can develop about the same amount of maximum lift, with the K380 being a lower aspect ratio lifting surface. The K380, being longer, can provide a little better control at high speeds with just the narow tip dragging.

There are a few other high-performance tab packages available besides the Merc's. Unless you get a set used, you can probably do better.

do you have a preference on a tab manufacturer?

My firsthand experience is all K280 and k380, but I have been eyeballing these for a while:

http://www.offshoreonly.com/classifi...o27131-en.html

Perhaps someone with some experience with these could comment?

I designed and built some carbon fiber ones a few years back with the intention of bringing them to market, but put it on the shelf when I saw where the powerboat markets were headed. I still have the molds, and a couple prototypes in various stages of completion. Someday if I have time, I will finish what I started and get them out there. They'd make a nice addition to a carbon boat...