I would think the peep would be too high for the front sight if you had it mounted on the MAC1 mount. You can always JB Weld the Williams 5D-SH on if the gun isn't drilled and tapped for a peep.
The Mac1, IA mount isn't made for a peep sight. It's made to mount a scope. That's why it called Benjamin/Sheridan IA Scope Base.http://www.mac1airgunshop.com/product-p/bjshia.htm
Pumping is not bad holding it by the grip as I pump it
The intermount doesnt put any stress on the barrel
Now I wonder why that is. Did you know Ive been doing this for 5 decades with my sheridans and never had a problem doing it this way. The manual picture is just that, a picture showing an unscoped dan being pumped,nothing more. Its ergonomically easier that way of course. But I love the results of a scoped Sheridan and willing to give up some ease of pumping for the few shots Ill ever need.