sooo... a 518 or 46RH with probably *checks notes* a JW ultrabell... and all the associated upgrades.
i know that around here, 518's and 46RH's are hen's teeth. so once you've factored in the price to obtain a core and then the price of repair components, upgrades and a rebuild (if you can't do that yourself), plus a full custom converter... you're easily into the 4L/2004R territory that enjoys much much broader parts availability and aftermarket support.