Mike,
i see now what you were saying from the picture. If you want to replace the 1/4" and make it so that you can go from bow to stearn on the bottom you can scarf two pieces of plywood together too. You can scarf the two pieces in a number of ways. Scarf joints are 8 to 1 slope. So you can draw that out on the plywood and then use a block plane or belt sander to create the slope. You then epoxy the joint together. now you've got the desired length of plywood to cover the entire bottom (i agree with you about the 8' of 1/4" creating some drag). i'll also add that a scarf joint with epoxy is actually as strong or stronger than the plywood itself (i've done the tests and the wood breaks before the joint does). I have a skill saw set up to cut scarf joints too, makes cutting them quick and easy if you feel uncomfortable doing it the other ways (i'm just up the highway from you in bellingham). anyway just a thought. and if you can't find the correct lenghts of oak you can scarf joint those too.

And sense you mentioned "wood jet sled" hear is a link to where you can buy plans for a wood jet sled.
http://www.tracyobrien.com/showcat.asp?id=2
I built the headwater 16, but look at the deadrise 19 if you're interested in a sled.

As a side note, i fish the cascade quite a bit in the fall so maybe on one of those trips i make up there this fall i'll have to stop by and take a look at your finished project.

Take care,
wingnut