Ive read a bunch about 'modern' cranks on several forums, and they look snotty, are lighter (by by ounces), and are supossed to spin easier......emphasis on the 'supposed to'. An assembly spinning on good bearings will spin, just the lighter crank will be a little easier and (by split seconds) quicker to get to speed, after that you are just maintaining rotation. On an older bike with a good one piece crank I dont think I could really tell the difference, the physics of it just dont show it to me. Tires and gear ratios do way more for me in ease of pedaling.
I would build that bike up with what you have, or can get reasonably, use new bearings, and use it for a year, then reconsider what mods to make.
