Those brakes are going to be a disappointment... but, ESPECIALLY if you are trying to brake from on the hoods. The old style levers for one thing just did not offer the same mechanical assist that newer Aero levers give, simply because of their design. However, a few things come to mind that can help you.
1.) First. change those nasty old pads! I'm sure they must be in poor shape, probably dried-out or greasy just because of the age of the bike. Jagwire® makes a pad & holder replacement called their "X-caliper" pads. These are identical to the standard 1970s-90s Weinmann "WX" pads - which Weinmann just fairly recently discontinued. They work fine, and they are perfectly sized to fit those particular calipers. Better still would be the old red compound Scott-Matthauser pads which I believe Rivendell has still been selling for around $20 per set of 4. They use the same patented compound which KoolStop eventually bought the recipe for.
2.) Check out, clean, lube the calipers. They are probably dirty at very least. Make sure all the pivots are lubed so they will move smoothly. And make sure the springs are sliding smoothly where they slide against the caliper arms. Also lube the straddle wire where it passes across the little hanger. AND, Make sure the pads are lined up so they are contacting the the rims evenly and and as fully as possible. You don't want them to be touching only at one small point on the rim. If the caliper arms are not pressing them down flush all along their length, you an actually (gently, please) grab the arm near where the pad attaches with an adjustable Crescent-style wrench and bend it as needed. Park® makes a tool for doing this, but it's just a simple small steel bar with a notch cut in it, so you really don't need anything sophisticated. Just be sure the calipers are firmly bolted down to the frameset, and then go very slowly, and keep checking your progress.
3.) You could always change the levers over to more modern style "aero" levers, and this will make a noticeable difference in performance. Shimano makes the R600 levers which look real pretty and are very comfortable, and they offer a familiar feel to the modern hand. But, at very least, you should probably change both the cables and the cable housings. You want them to be moving as friction-free as possible so you're not fighting that resistance as well when you're squeezing whatever levers us use. Nice modern teflon lined cable housings and "stainless steel" cables really can make a difference. I mention stainless steel cables specifically, because if you look at cheaper galvanized cables under magnification you will notice what appears as bumpy irregular deposits plated onto the tops of the cable wire strands... which means, just more unnecessary friction to fight against.
4.) I would recommend you do some serious cleaning on the rims themselves. chances are they have years of built up dirt, grease, road grime all dried on. I'm referring here to the braking flats. I will often use a Scotchbrite® plastic scouring pad and Rubbing Alcohol to free rims of any contaminants. Rubbing alcohol rather than detergent, because many detergents and soaps have everything from lubricants to keep your hands pretty and moist to fragrances, and well, just lots of crap. The Isopropyl alcohol is commonly used for cleaning tapeheads on tape recorders and even the surfaces of CDs and DVDs where you don't want any deposits of anything left, so you know it about as free of impurities as anything you can find.
5.) [Be advised, this next is going to make many List Members hair stand on end] Personally, I would recommend you do this on any used rims: I suggest you actually lightly hand "machine" the brake flats on your rims. I would just get a sheet of maybe 220 to 320 grit emory paper, put a piece of it on a flat piece of wood or a sanding block - actually even a wooden ruler would work fine. The idea is to just follow around the rim with it to remove any irregularities like nicks or even bumps at the welds and leave a freshly roughed satin-looking surface. You could even just go to Walgreens and buy one of those double-sided black fine-grit emery board nail files (around 1" x 6") they cost only around $1. You do NOT want to remove a lot of metal, nor even to abrade completely down past any existing grooves from years of brake pad wear. You just want to be assured of a somewhat flat & fresh breaking track to start out with... Be sure to clean away any of the residual dust left on the rims - again, with alcohol. THEN use the new pads, and they won;t be prematurely worn away from any pre-existing nicks on the rims which may have actually pushed UP a sharp berm of aluminum.
--- I still use Weinmann (and other) center-pulls on many bikes and they can work very nicely. Used with aero levers mounted on moustache bars they really have absolutely devastating one-finger stopping power, so you should still be able to make them work very satisfactorily on your bike. One habit I've gotten into after a long dirty or wet ride is to clean off my rims and pads right after riding so they'll be all ready to do their best work on my next ride.
Good Luck!
Bob Hanson, Albuquerque, New Mexico, USA