The 4.0 will take a lot. I put 203,000 miles on my 2006 - with nitrous
followed by the Xcharger at 90,000 miles (not both though, it won't last with dual power adders).
No need to forge the engine - there are many many many of them out there with lots of miles and lots of boost with no issues.
The limit that my friend found was when he hit it with 18lbs of boost and about 500 rwhp. Another friend is running a turbo with 16 lbs of boost and 475 rwhp and has been for some time now. Again, stock engine with just the valve springs changed to avoid valve float.