This is one of the things that I spent some time on this summer. Although my motor is a 216, I think the problem is similar.
When I rebuilt my 216, I did it in place because I didn't have a choice. As part of the rebuild I installed a new rear main seal. I didn't replace the main bearings. The seal lasted for a few hours and then began to leak. This summer I replaced it with the Best Gasket product which is teflon based. That seal lasted for about 100 miles. On my next try I replaced the main bearings. As it turns out they had way too much clearance. On the 216, shims are used and I was able to adjust each main for the minimum clearance. I installed an NOS rear main seal and it has held up really nicely.
My point here is the problem was excess clearance in my main bearings allowing the crank to move around. I also think that the Best gasket product is a superior product.
