Hi Metal Man,
It sounds like you haven't completely committed to building an RV-7A yet. I think it's smart to ask these questions before you do. I happened to see your question, and thought I'd comment because this is something I've had some experience with (not the modified canopy, but the seat cushions). Our company has been making seats and interior items for RVs for about ten years now, and this is one of those questions that comes up from time to time.
This mainly pertains to the RV-7s and 9s (it's not usually an issue on the RV-8). More than once, we've had a tall customer who built the whole airplane, and assumed that by making the seat cushions thin enough, they would have adequate headroom. The RV-7 and RV-9 are not small airplanes, but they are not big either.
Of course knowing your height is only part of the information that determines headroom. You also need to know how much of your height is in your legs vs. your upper body. I think the easiest way to determine if you will fit is to do a simple test in the model of RV you are considering. You can just take out the lower seat cushion, and sit on the floor. If you plan to wear a standard headset, put it on for this test (you might consider one of the in ear headsets that don't add any height on top of your head). Now measure the distance from the highest point on your head to the canopy. From that number subtract whatever clearance you would want to avoid hitting your head in turbulence (1" is usually a good number). The distance left over (if any) is the thickness that can be allowed for your seat cushion. If it's less than about 1.5", you won't have much of a seat cushion.
I hope this helps. I just saw your last post, and it looks like you might have all this figured out already. Let me know if I can answer any other questions about tall people and seat cushions (I try to read the forums regularly, but I don't always have a chance).
Best regards,