Yeah you snowbirds run out of tesing time a bit early up there....

However be thankful that you have to winterize because that usually means that you have a boat that actuallty runs unlike myself.
Not sure what Stainless Marine would get to lengthen your risers , But you may want to check with Brad ( funhome ) here on the board. I know he changed his risers over to dry by cutting his existing Gil risers & adding to them. You may not be looking to go completly dry but it would be pretty much the same process. Send him a PM he's a great guy
My point is that a Good ( keyword being Good ) fabricator that is handy with a TIG & is good with stainless can handle the job.
Just a thought .....
By the way if your #'s are correct & that 24 pitch spinning at 5,000 is 68 MPH on GPS ( which is I believe what you said earlier ? ) That comes out to 11% propslip wich is really good for a 28 Single.
Talk to you later , Scott