Thanks guys
John I have been looking at Bjorn Thomassons Panthera . So I am really glad you commented . I had already thought i would stretch the forms out a bit bringing it out to 580cm .I and overall need just a bit more room to fit my feet and squeeze my gear in for my longer trips

The Panthera holds more gear than I'd thought when I first looked at the plans. It's a quite capable touring boat and I think it could carry gear,food and 3-4 days water for a trip of a couple of weeks easily.
The 403L volume is big - though I've found it's best to leave the extreme bow filled with something very light (or some bubble wrap as a spacer). You'll need a stick with a hook to retrieve items from the ends; I do.

I don't think lengthening Panthera a bit would cause any problems.

Putting the fwd bulkhead in 'the right place' (i.e. far enough aft to give good storage in bow compartment, but not so far that you can't get the right leg position) is something I pay more attention to, lately. On trips, the space in front of the footboard/footpedals (if any) can be used for spares that hopefully won't be used : spare sprayskirt, drysuit gaskets, etc..

Here's Bjorn Thomasson's page with info on altering designs (Gist: Length OK, others not):
http://www.thomassondesign.com/en/building/alterations