: Has anyone modified a Yost design to the extent
: I would need to?

: I have also looked at some stitch-and-glue kits, but I am bit leery
: at committing to a more expensive model without the option to
: customize it and without having tried any of them.

: I would appreciate any advice!

I've built a few kayaks- both wood-strip and S&G (no SOF boats), including one Pygmy Arctic Tern 14 kit.

A few thoughts:
Unless you know exactly what you want/need, and can't find it in an established design, I'd recommend against modifying a design for your first build.
There's a pretty good selection of designs (and kits) 'out there'.

If there is any way you can paddle 'a bunch' of boats (other builders in the area? kayak get-togethers, renting commercial boats, etc), it would help your selection process a lot. There are a lot of Pygmy boats out there, so you might find some in your area to try. (or take a west coast holiday to Port Townsend... ;-) )

Kits: If you were building a S&G boat with marine-grade plywood, you'd probably find that the cost would be a good fraction of the price of a kit. And you would save a lot of work. Using kit panels is much less trouble than laying out and cutting them (accurately) yourself.

I can sit on the floor with my legs straight, but it's a 'whole 'nother thing' paddling like that for a few hours, for me.

What's your budget?
How much time do you want to spend on the build?
Have you decided to build a frame-type (SOF) boat, or are you also considering strippers and S&G boats?

Duane Strosaker's Point Bennett is a nice looking kayak which should have pretty solid secondary stability. It would be at the top of my list if I were building another S&G boat.

http://www.rollordrown.com/kayak/index.html