If you're still working on this, could you add a close-up of the attachment points on both the headboard and the new frame, plus the frame's make/model? Measure the hole spacing and height from the floor on each, too. That would help distinguish a simple alignment problem from two different attachment systems.

I'd avoid drilling the antique brass or buying a 'universal' bracket until those details are checked. Ask the frame manufacturer which headboard attachments it supports; if the old fittings need adapting, a furniture restorer can assess a reversible solution. The mattress and its frame should remain properly supported independently of any improvised headboard connection.