Drop an e-mail and I'll send you a spread sheet, or you can access the weight and balance worksheet that Van's provides on the company website (
http://www.vansaircraft.com/pdf/RV4wb.pdf). About 450-475 for each main weight and 45-50 lbs at the tail will give you a reasonable average to start with. A basic operating weight (everything except bodies, baggage and usable fuel) of 975 or so will give you some flexibility on loading and remaining below designer's recommendations for max gross. There's enough information on the drawing to derive approximate station information for the propeller/flywheel locations for a cocktail napkin WAG. Van's recommended datum for the -4 is 60 inches ahead of the wing leading edge, so all moments are positive. Empty weight varies widely, and it's not uncommon to see a -4 that exceeds 1000 lbs empty. Assuming a 1K lb airplane, 192 lbs of fuel aboard, 2 standard FAA 170 pounders, you've already exceeded the 1500 max gross recommendation. Even a single 170 lb pilot with full fuel is close to the aerobatic gross limit of 1375--just about there with a 13 lb parachute (if one is worn).
There is only so much leeway for manipulating RV-4 weight and balance (hence the RV-8!). I'm a "light nose" fan for handling and performance qualities, but it does limit how big a person I can reasonably put in the back. I'm just fine with that, but I can also understand that limit may be unacceptable to some. Like you said in your original post: Built-in lightness--the key to overall performance...A heavy nose can increase the CG range, but has a detrimental affect on gross weight--there is limited trade-space.
A couple of accepted solutions, shy of modifying the engine mount/cowling are to utilize a mass ring bolted to the starter flywheel on the engine or to have a custom made crush plate fabricated that puts weight right at the propeller. Saber manufacturing, the folks that make the fixed pitch propeller extensions can fabricate crush plates to your weight specification. One example of a mass balance on the flywheel is a Landrol hydraulic damper--they will pop up from time to time in the classifieds. There is also quite a selection of engine/propeller combinations that can help you manipulate CG (e.g., light engine, heavy prop, etc.). I think it was Van that said Lycoming never built the same engine twice! Add to the mix all of the "after market" engine combinations/components (as you mentioned!), and there is a wide band of suitable powerplants and propeller combinations.
Moving the engine sounds interesting, but there may be some unintended consequences. It would be interesting to look at the data after you're done testing! Keep us posted on your progress.
Best of Luck,
Vac