Yep, Ingalls "594" bushings. Not sure all the adjustable bushings are equal. When I visited Hendersons LineUP, they mentioned Ingalls and Specialty which is Specialty Products Company (SPC) and the E-Series part is 24180. Harvard has done a lot of research on alignment and he cautions us against "just any" brand. Based on his remarks and successes with Ingalls, plus Hendersons' comment, I think I'd consider only Ingalls and SPC.
The "594" and "24180" will fit E450, no problem. They aren't listed in catalogs because the manufacturers don't trust them to stay put in heavy duty applications like cube vans and RV's. I've had two shops tell me that if they find them they replace them with fixed. Mentioned that at Henderson and they said the bushings will stay if the torqueing instructions are followed. I'd add, de-grease all surfaces and use Loctite.