Honestly, as much as I hate to rain on the parade, I WOULD NOT DO THIS. The shell of the helmet has most certainly delaminated (probably severely), or at the very least undergone chemical changes which have left it brittle as your great-grandpa's OEM pelvis assembly. That nice, new liner you're planning on assembling won't do much except form a nice, soft bag for your grey squiggly-bits if the force of an accidental impact isn't dispersed/s[read/absorbed by the helmet shell properly, a shell, let's not forget, that probably wasn't all that great to BEGIN with given the vintage.
If you really want the vintage look, there are plenty of DOT-approved modern replacements made from original Bell and other molds, many with old-skool fat-flake paint, fittings for the old "face bubble" sheilds, etc., and THOSE won't shatter on impact...
-M