by "lengthen coaming" I infer that this is a completed boat or are you building and have not cut/mounted said coaming?  Also, to be clear, since I am sure others will ask, is this a C18? (I'm guessing so)

I am interested what people have to say as I am about to start my C18 with an enlarged coaming.

I have a 3 yr. old Ches. 18.  For tall folks this cockpit has to be larger. 
Tough to get out without being able to use your legs.  I plan on making it maybe 4" longer.  I reckon I'll just hack away at the old coaming and then sand it flush to the deck.  Make my marks and cut away.  Then rebuild.

Seems like people with long legs are ignored in the design of kayaks.  

Kim

Just chatted with a CLC guy and he will make a coaming printout 4" longer for me.  That will make it easier. 

Kim

 I am part way thru my build of a CH18, and yes, you only buy an 18 if you need the space :)

I have actually used the cockpit ring sort of as a template to make one that will "fit me"  I made mine 2 1/2" wider and 4"  longer, finishing at just over 34" opening length.

Whilst I am only about 6'1", there is no way I could see myself getting into the standard supplied cockpit - Seriously, a larger opening should be an option for this boat design - Big Guys buy this boat!