Hi Tom Unfortunately, your 8 X 8 sill plate has long outlived its normal useful life. In 150+ year old houses, rotted sill plates is probably (at least from my experience) the most common cause of uneven house settling, resulting in doors and windows not working and floors that slope.
The only solution is to unweight (not jack up) the sills and replace them with pressure treated sill plates made especially for this purpose. Although the beams may be mortised into the existing sills, painful though it may be, I'd recommend cutting off the tennons and reattaching it to your new sills with beam or joist hangers. Concrete fill between the sandstone and whatever is resting on the existing sills is a best a bandaid. Pouring anti insect, anti mold stuff into the old sills is, IMHO, a waste of time.