Replacing 1963 Valiant starter

Hmmm, interesting. Why would you want a "more powerful" starter? Maybe it's possible yours is not getting full voltage from the battery which means you have a voltage drop somewhere due to resistance in the wiring or connections. Check the big fat red wire's connections all the way from the battery to the starter. Make sure they're all tight and clean. Also check the black ground wire on the battery and make sure it's grounded properly to the engine. You can verify voltage drop (or not) by using a voltmeter from the center of the + battery post to the starter post. Check for a voltage drop while someone else cranks it. It should be less than .5VDC.

If it's still not working right you may have a dragging starter in which case you should simply replace it.