I have a Highfield 310CL and it fits perfectly, fully inflated, bow facing aft on the aft deck on passage. Clears the mizzen traveller and the tubes don't intrude into the cockpit space.
The disadvantage is that it's barely long enough to fit on the Simpson davits (with Suzuki 20AS motor on) so the motor rubs a bit on the davit arm.
I think the Highfield 340 would be too long to comfortably fit on the aft deck without deflating the tubes. Given that's its resting place on anything other than short daysails in calm weather, we opted for the 310.
Hope this helps.
A54 #69 Tengah