I think goals of 100K miles, any sort of mileage, and 500hp/550tq are themselves self-cancelling. Factory engines had some issues reaching 100K miles. Mopars were great at it, but most after 20 years still only had 80K. If you plan to drive cross country on a regular basis maybe you'll get that high, but I would think you should re-appraise this goal. Gas mileage should be the dead last thing you worry about with the 1hp/inch engine. If you can't afford to fuel it, you can't afford to build it. So agiain, I'd say change the goal to simply accept whatever you get. Personally, I don't believe you could get better than 12-15mpg with a 3500lbs car, mild gears, and an overdrive trans or unit. lastly, the horsepower level... A longer stroke makes tons more torque. It also loads the block's main webbing more. Anyone with enough history of 440s around or over 500hp will tell you the weakest link is the block. Beyond that, camshafts designed for bigger engines wear things like valve springs much faster (comparatively speaking), shorter pistons are less stable so rings and pistons wear out faster... Basically, I'd say build what you want in terms of power, and for the life span and ,mileage, take what comes. The 496" RB (4.15 stroke, 6.76 rod) is a nice package and uses a taller piston which goes a long way to helping life and ring stability. The 505"RB (4.25 stroke, 7.10" rod) has a short piston and I wopuld not expect much more than 20K max from it. If anyone has legitimately gone further on one, I haven't heard about it. Maybe someone has... I doubt it. I do have personal experience with a 496 that has had about 25K on it, runs 11.30s in an E body with 4.10s, and smokes a little but runs well. (also runs a street solid roller cam). If it were me, I'd look into the 496" engine. I would not trust 440Source balancing, and thier kits will need work to make right, provided you have the initiative to really inspect the parts properly. They are still IMO the best value, but I've never had one that was good enough to simply drop in.