I have been looking for an underspin that can handle larger paddletails like Swing Impact Fat 5.8s and 6.8s. Anyone know of any out there?

Only thing I know of is the owner beast flashy swimmer. 

Found one, comes with a 5,6, or 7/0 hook

I think large underspin is a bit like jumbo shrimp, but you can rig a second hook on the first on a smaller head, just gotta make sure the body doesn't interfere with the blade. I do it on small chatter baits with 7" straight tails, when fished with heavy mono, it sinks slowly, almost suspending. 

Trokar makes one up to 7/0 for a reasonable price.

 

https://www.basspro.com/shop/en/eagle-claw-trokar-swim-blade-jighead?hvarAID=shopping_googleproductextensions

 

 

Basstrix Spintrix paddle tails 6" & 7" with Blade Runner Spintrix Underspins 3/4-1 oz, outstanding combo.

Tom