Mike, just so you know before you use the Xtreme Xtension it will leave marks on your bit shanks as shown in the photo. The Musclechuck will not do this and gives you 4 times the holding power of the factory collet and nut.
Just so we are on the same page you did remove the sub base plate before installing your router? I think it might be better to consider flipping your top upside down and using a template rout the top down to 3/8" thickness where the router attaches.
Another thing you should know is that Musclechuck is working on a design for an extended version of its chucks that will provide about 2-5/8" additional depth of cut.
"Living in the D" (this means Detroit!)
"It's fine to disagree with other members as long as you respect their opinions"