The rubber (people call it neoprene but it was more close to viton according to what Fel-Pro told me years ago) is the best option unless you're going to have the block machined as Jerry mentioned. If you use the rubber 2 piece seal be certain to use a Speedi-sleeve to restore the rear crank surface. It will very likely be chewed up and if it is, it will not work right. If you have the crank ground and that rear area polished, you'll need to shim the rubber seal to make it seal correctly. You'll probably need to add .006" to .008" underneath it. Also if you have the crank ground, make certain the work is done so that the bearing clearance is right on the lowest of the factory suggested clearance. Which is .001". That will keep that long wobbly crank from moving around as much and keep as much oil from shooting out of that bearing to crank area. Good luck!
