[Edit] Using your A3K Agnus should also give you the ability to jumper between PAL/NTSC booting.
Nope, the A3000 Agnus I have is a 318069-18 (PAL-only, needs a VBB fix). While well-known websites are falsely describing it as an AGNUS for A500+/A600, it is indeed for A3000. See the last page at this link
which explains how to fit in a A2000 (and so how we can fit in a A500 rev.6A with a probable VBB fix to do on JP4). On the other hand, the 318069-19 is correctly described as an NTSC only Agnus for A3000 by those websites. That was the reason why I thought something was wrong with the description of version -18 and finally found out a German document listing all of them which says a different description for -18 and then the document I linked here.