My existing (original) arrester belt (above the front snout of the diff) seems to be in good shape except for being a bit stiff and dirty. Is there any trick to help return some suppleness and flexibility to this?

Here is some threads that may be helpful for you to recondition the strap.

https://www.google.com/search?q=240z+differential+belt+recoditioning+classiczcars.com&sca_esv=cc17e54ac3639420&rlz=1C1VDKB_enUS1121US1121&ei=17rqZ4miB5qLwbkPiPnusAk&ved=0ahUKEwjJ6dOL17SMAxWaRTABHYi8G5YQ4dUDCBA&uact=5&oq=240z+differential+belt+recoditioning+classiczcars.com&gs_lp=Egxnd3Mtd2l6LXNlcnAiNTI0MHogZGlmZmVyZW50aWFsIGJlbHQgcmVjb2RpdGlvbmluZyBjbGFzc2ljemNhcnMuY29tMgcQIRigARgKMgcQIRigARgKMgcQIRigARgKSJY6UPEFWIg1cAF4AZABAJgBuQGgAZUSqgEEMC4xN7gBA8gBAPgBAZgCEqAC2hLCAgoQABiwAxjWBBhHwgIHECEYChirAsICBRAhGKsCmAMA4gMFEgExIECIBgGQBgiSBwQxLjE3oAeBdA&sclient=gws-wiz-serp