Surfer,
Changing the bars and seat position on any bike will effect the way it sits but not the ride. The smaller (skinnier) the tire the firmer the ride. If your bike is the traditional 10 speed racer I have to assume it has thin tires. The cruiser style bikes with fat tires typically will be more comfortable on old rear ends, like ours, and they allow for a more comfortable sitting position. It has been my experience that most bikes will benifit from a lay-back seat post. This moves the rider further to the rear and gives a little more knee room and leg stretch. Hope this helps and send us pictures of whatever you decide to build.
Tom

__________________
Age and Treachery Will Always Triumph
Over Youth and Skill