SBP to BBP brake clarification

Can I buy a new big bolt pattern disk and swap it with the small bolt patern disk? Does everything else swap over?

71 Valiant /6 Factory manual front disks.

Nope. The spindles are different. Wheel bearings are different sizes, the track width changes between the two.

Changing over to the 73+ BBP disks requires changing over to the later spindles. Which requires different calipers, caliper adaptors, a new lower ball joint and either upper ball joint adaptors or the later large ball joint upper control arms.

Wouldn't drilling for 5x4.5 be easier

Not at all. Separate the 2 piece hubs, machine them, add material to support the new wheel studs because there isn’t enough, etc, etc.