Hi Fergus, I can recommend the nomad 8.5 which has 78 gallons and a max weight of 240 pounds. Or the dagger mamba 8.5 which is the boat im currently using which has a max weight of about 260 pounds and 72 gallons , though it is a little lower in volume its a grand boat and great for bigger paddlers ( me being about 6.2 and about 19 stone).
I ran it down the clydagh a few weeks back when it was at about 1.5 on the gauge and the gates and it served me grand even though I am pushing the max weight and had it weighed down with kit.So it'd serve you fine considering the weight difference.
Feel free to message me if you have any questions about it.

Dan

Fergus,

I'd probably swim in the Colligan myself and get spat out and regurgitated on the Clydagh, but its all good fun.

I paddle the fluid solo medium , it perfect for the bigger volume rivers , im going to test it on the coligan next week so i can give you my review ,
the large or expedition is wider so that might fit you and it is has a huge amount of volume , but it is a creeker rather than a river runner so its not as fast as a wavesport desiel or LL remix