Modern long reach calipers on old bike, strong enough for safe braking from the hoods?
I got the Shimano  BL-R400 levers for my bike and they work great with my Dia Compe centre pulls brakes so I see no reason why they wouldn't work with your Weinmann brake which is similarly designed. 

Using the center pull brakes combined with the new levers provides ample braking power both from the hoods and the drops. I can't speak as to how well the braking would work for modern calipers. 

That being said, the BR-R451 calipers are only "long reach" by modern standards. There are calipers with much longer reach available if you need them. The braking power shouldn't really be effected too much by the length of the caliper.  A longer caliper arm will tend to flex more, but the arm on the BR-451 isn't even really that long.  They have been designed by a quality manufacturer who would taken into account the longer arm and design the brake with the right type and amount of material to resist excess flex in the lever arm.