bridgit -- Welcome!
Yep. IKEA's Blum boxes are proprietary, and can't be mixed with regular Blum hardware.
We too have a strange, old kitchen, and we ordered
a single drawer box from Scherr's to fit the spot, then cut a drawer front and painted it to match. It's different from the Tandemboxes--it's a dovetailed wood drawer, but it doesn't bother us to have different drawer construction.
Anyway, yes, if you want Blum and in a different size than IKEA offers, you'll have to pay the price. Sorry!
Susan