Shimano 600 Brake pads

Shimano shoes are same shape from about 1990 onwards, so you can just stick the coolstop inserts in them. Salmon are rumoured to be the best, about to order some myself to replace some jokes ones I got from CRC, which were super soft and decent when new, but 1yr down they line they are like blocks of hardened plaster, useless, just make a mess of the rims.

Actually I'm pretty sure you can't just swap out the old pads from the 6400 holders (similarly those on the 7400 DA's)
IIRC the fitting isn't the same, the older pads were wider.

In any case the old pads will work just fine, I'm still running the original 600EX pads on a 1986 touring bike and even with the 600 single pot brakes they work superbly..That said if you're that worried get some modern holders and get koolstop, Salmon as mentioned maybe look for a dual compound type. However unless your rims are shot to buggery most decent pads will work.