I like that you don't mind taking a risk to get a boat you want , you will learn a lot.

at 14' i would look at measurements of similar length boats to get an idea for beam etc. Not sit on tops because they are wider
for more stability because the paddler sits higher.

My way to find the seat placement on a new self design is simply to glass the hull first..sit in it on rolled towels
and balance like a sea saw on those. but guess what thats not same as being in the water so before
you do the cockpit rim, with deck attached or at least taped to the hull, sit in it in water...and experiment a lot with moving your weight fore and
aft and paddle it some. Pics of someone in your boat might show too much weight forward or aft by looking at the waterline...actually that is what helped
me most last time...did'nt look right so i cut the cockpit opening 6" further aft...""wow new boat""

In my case on my last boat it was the difference between a boat i would barely paddle to the only boat i want to use. sweet spot.

Don't break the boat sitting in it unfinished but do what you have to. And don't be afraid to hack away at the deck to move a cockpit.