ahh im having the same problem on a townie bike build ive been working on too long.
currently i am searching for a later model 700c fork made for short-reach brakes. since i already bought a brakeset. personally, i think that would be the best fix, since you can probably pick up decent short-reach brakes fairly cheap off ebay or bargain bins. i got a set of veloce brakes for $30 shipped.
Just sold a set of r556’s to a guy yesterday who had a similar situation. Measure first, of course, but those should be a swell solution. And at only 17 bucks or so a piece, they’re a fine value, I reckon.
On my ex-conversion (a late 70s Nishiki International) I had to run 700c wheels in 27" spaces; I had the same gap as the OP. The Tektros didn’t reach, I had to get the Cane Creek SCR-3Ls (or something like that, it’s a long version of a standard Cane Creek caliper.)
Edit: here’s a pic so you can see the gap I had to make the brakes reach across: