The replacement seal may not have the same 'profile' as the original, or the material may be a different composition as you stated. You might be able to adjust the side glass slightly, to eliminate/reduce the leaking problem, but its 'project' that you may or may not want to perform, as it involves a fair amount of worked to access all the side glass adjustment points. The widow adjustments are covered in the '70 Body Service Manual. The alternative is to stay away from high pressure car washes !!