Replacing old brake blocks with modern pads I'm looking to freshen up a 60's Raleigh Sports that has the old standard brake blocks on it.  If I able to put modern pads (http://www.koolstop.com/images/ks-bmxdl_art_lg.jpg) from a cantilever brake on it, as long as they physically fit the rims, will I run into issues with braking forces causing extra stress?  Any issues with performance?  

The bike is all stock with side pull brakes and levers on it.  Wheels are original steel and seem to be true (not checked in a stand).  Fork, frame, and wheels are in great shape.

Side note/discussion:  Koolstop carries the blocks (http://www.koolstop.com/english/continental.html) as well marketed for such a bike, recommended or not?
Those long "mountain bike" Kool-stops work fine on my modern skinny wheel road bike with side-pull calipers. If the modern Kool-stops fit (and don't see why not), I'm sure they will outperform those vintage looking ones - the contact area is much greater. I'd even bet the long contact patch results in less torque on the caliper arms, but I doubt it's an issue either way. 

Those Kool-stops are nice, the mounting hardware allows fine adjustment of the plane of the brake surface. You're supposed to adjust them with a little toe-in (toe-out?) so the inevitable torque when applied straightens them out (we're talking a very small amount of inevitable torque twist - but the Kool-stops are designed to account for that). That little bend on the tip of the red side helps with the setup for that.

Take the bike in and ask the bike shop, but my guess is they'll work fine on 60s era calipers/skinny wheels.