You're going through a lot of the same process that we've used to determine the aero shapes on our bikes. What we've found to work well is to make full size templates of side profiles and frontal areas with the rider in riding position on the bike. We used large cardboard boxes, like a water heater or refrigerator carton, flattened out and propped up beside the bike. Have an accomplice (it's always good to have accomplices, or even minions, that way you have someone else to pass the blame onto if it doesn't work out exactly right), trace your outline on the bike in full riding apparel and position, and the bike outline onto it in plan view.Top, side, and frontal views. That way you can draw out the shapes that you think may work on the cardboard to really visualize them fully. I don't know about you but my mind works best in full scale and 3 dimensions.
That's the process that we used to develop the shape on the 7419 bike. It seemed to work well for us anyway. Here's a picture of it right before we loaded up to leave this year.
Of course, as everyone knows this body is wood. We were running out of time and I wanted to at least prove the concept for the bike, this year with the chassis and engine mostly done I'll be starting early in the spring to make a real body for it, more in line with the original concept of some sexy fiberglass, like so.
I kind of went to extremes to minimize frontal area and maximize the tail length, and was concerned that the large slab sides would make it uncontrollable in cross-winds, but it turned out to be a complete non issue and rode very well through some side gusts over 10mph, so I feel comfortable putting the effort into a more complex streamlining setup.
And yes, Tom Mellor's bike is a very nice piece of kit. I told him personally that his body work was a great inspiration to me in the design of the rear end of mine. I don't know if he took that as a compliment though, LOL.