[lolife99]

Belltech 3121 is the correct spindle to convert a 63-70 drum brake truck to disc brakes.
I don't have balljoint numbers,... But the 71-72 and 73-87 trucks share the same LOWER ball joint. The uppers are different as are the outer tie rods.
Any chance your balljoints have been changed?
I assume you are converting a drum brake truck,... And not just adding DROP spindles to a 63-70 truck already converted to discs?

Any pics would be helpful,..