I may be a bit late to this question, but just in case it is still open...

About the only lowering of the seat in the OTS that can be done is to remove the spacer under each of the seat rail mounting points (at either end of each rail). Then the seat rail can be bolted directly to the floor. But that would only gain about an extra half-inch of head room. Plus it may interfere with the slider action so that the seat becomes essentially fixed. And the carpet under the seat could not be retained - maybe just a sheet of color-matching vinyl? In any event, the front of the seat could not be dropped much more since there is a frame cross member that passes directly under that part of the seat.

Maybe the Series III is the only option? At this point, any of the E-Types are getting rather aged, so I don't think that maintenance and expense should rule the V12 out of hand in a general sense. They ALL require maintenance and expense to live with. A very good V12 might be a lot easier to live with than a ropey 6.