I've run Dura-Ace 7400 brakes; I probably have a few sets sitting in a big bin 'o brakes.
The problems with these vintage pads is not just they were too small, and the rubber is hardened after 30 years - the main issue is that for a multi-decade span, including the 7400 era, Shimano's brake pads were pure crap. They had a unique ability to pick up small rocks and metal shards, which when embedded, proceeded to grind away at your rims. In the rain, in addition to the scary inadequate braking, the pad material turned to a black goo that coated your rims, and with centrifugal force, flung it out onto your person.
Shimano pads are now much better.
For even more improvement, I recommend ditching the 7400 single-pivots for the 7403 dual-pivots. Much better braking and they self-center. The 7400s were the best single-pivot brakes ever made: superior mechanical advantage, excellent materials, hardware and finish. The 7400's are in every respect better than hopeless vintage floatsam such as Gipiemme, Ofmega, Modolo, and the worst: Dia-Compe. When I receive these at our local bike Co-op, they go deep into the trash. Flexy, thin arms, overly stiff springs, and cheap awkward hardware. Life is too short to be riding on poor brakes - unless you want your life to be short!
Nevertheless, the braking power of the 7403's are far superior to the 7400s. Option: the 6403 dual pivots are just as powerful, and can be bought on Ebay cheap cheap.