Reasonably priced BIG BRAKE alternative that fits in a 15” wheel

those sumitomo calipers that "inertia" Posted off the hilux and lancruiser bolt on more or less without a bracket if you use small rotors
they are a favorite upgrade caliper for the datsun/nissan 240 280 z people

They use a 3.5 inch bolt mounting spacing
like many Wilwood, one of their calipers is nearly an aluminium clone of this caliper
like mopar bracket based calipers , like Austin, like jaguar xj

details here disk equipped 14 inch 15 inch wheel A body update to 4 pot calipers
landcruiser caliper upgrade - Mopar Muscle Association UK

and here
using a bracket to cater for bigger rotors
Anton's 68 Charger - 2012 Update - Page 7 - Mopar Muscle Association UK