I have a 505 Stroker in my 1971 Challenger. I put a new viscous clutch fan and Mopar shroud from Mancini. Still overheating. I was doing some reading and noticed the fans should be 50% out of the shroud for maximum cooling. I have no spacers on the fan and the fins are right at the edge of the back of the shroud. Am I supposed to cut the shroud so more fins are visible?

No, I wouldn't cut the shroud, that isn't your problem. You need to double check the usual suspects. Radiator core might be plugged, thermostat could be a problem, collapsed lower hose, etc.

Uh, no.
Something's up with that: thicker core, fan clutch shaft too long, maybe wrong shroud ?
Describe the cooling system, what's been done or re-done, etc.
Also, what you mean by "overheating": running hot, puking coolant, or what ? When did the problem start ?

I’ve personally seen to many fan clutches take flight into radiators to suggest ever using them on anything that revs rapidly.
That being my opinion, if you choose to run one, what you’ve read is correct. Any shrouded fan will move more air with the blade tips out of the enclosure. I doubt that is the issue... but it is true.
First thing I would check is if it’s working. A lot of them have adjustable thermostats. See if the fan is moving hot air.
I’ve also noticed that many temp problems can be fixed with correct ignition curves.