528 delay scythe = Sword strap.
Reason:
You only need at least +16 StoreTP for TP and for WS, for a "true 6hit build/5hit rebuild." Using Sword strap would put you at a 512 delay and you would need at least +21 StoreTP for TP and WS, for a true 6hit/5hit rebuild, which is just the standard /sam,rajas,and brutal.
More STP/Grip reference here.
Edit: or Rose strap could be an option if you're going for a 5hit build but that requires more StoreTP and some very specific gear. You need 39 StoreTP for a true 5hit build (5hit/4hit rebuild) with a 528 delay weapon. Or a 37 StoreTP for a "6hit/4hit rebuild" (not a true 5hit build), as long as you land at least 3hits on Guillotine WS.