John, I am sixty lbs heavier than you, even if I am using a skinny boat, I need some length, or I will be sinking the boat beyond its design waterline. I generally go for something in the 17 to 18 ft range.

Et:
I understand your point. The data on 'pounds per inch of immersion' is very useful as it gives an idea of the change in waterline with added load.
Unfortunately not all the designs and kits out there provide that info.

Here's an example of the data that Mariner provided:
Mariner Coaster
Specifics:

Length: 13' 5.25"
Beam: 23.125"
Cockpit size (inside): 31" x 15"
Depth at front of cockpit: 11.25"
Depth at rear of cockpit: 9.25"
Center of Buoyancy: 52%
Volume (filled with water): 12 cu.ft. (90 gal.)
Volume (Nautilus program): Sea Kayaker uses Nautilus (all the volume inside the shape) 12.5 cu.ft. (93 gal.)

Hydrostatics with 150 lbs. added:

Waterline length: 12' 7"
Waterline beam: 21.5"
Draft: 4.0"
Prismatic Coefficient: 0.51
Midship Coefficient: 0.76
Volumetric Coefficient: 0.00148
Wetted Surface in sq. ft: 16.37
Lbs./inch immersion: 75.2

Hydrostatics with 250 lbs. added:

Waterline length: 12' 11"
Waterline beam: 22.6"
Draft: 5.3"
Prismatic Coefficient: 0.54
Midship Coefficient: 0.785
Volumetric Coefficient: 0.00209
Wetted Surface in sq. ft: 19.62
Lbs./inch immersion: 84.8
Copyright Mariner Kayaks

And, boats with 'skinny ends' like the 'Brit boat' style (i.e. like many plans and kits) tend to behave badly when overloaded IME- they are often wet rides at the best of times.
And, popularity is no measure of a good design - the Current Designs Solstice is one of their most popular designs (skinny ends and wider flatter middle) and it's a wet slow ride in any sort of 'conditions'.

Nyah may well be better to choose the LT vs SLT, depending on the load he plans to carry in the boat.

Whenever the 'longer is faster' argument pops up, I see a red flag! Thus the long and argumentative response.....

I was very favourably impressed by the info from Vaclav at OneOcean. Bjorn Thomasson's designs come with a lot of data as well, and he has communicated extra data to me when I requested it. It's a pity that none of the current commercial kayak companies provide similar data (Mariner did, but they haven't produced kayaks for almost 10 yrs.). If you can access the old reviews from SeaKayaker magazine they have drag data and other info, usually.