A glass blower could weld a modern 18mm glass joint there, but other than that no, you can't make it glass on glass.
There are kits to sort-of make it glass on glass. It's basically a very thin downstem with a 14.5mm joint at the end of it. It works, but only in a half assed way. It'll be floppy and unstable, and you'll still be dealing with all the downsides of that rubber grommet.
Ok so for the time being I got a standard downstem as opposed to converting. That fits in the grommet but seems to butt up against the glass of the bong. My question now is if I put the rubber in place first and lube things up good will this fit?
Judging by the second picture, that doesn't fit. The grommet is going to add at least another mm to the diameter. The reason it fits around the down stem is that rubber stretches. Measure the diameter of the downstem you have now and get one that's thinner by 2mm or so. If you don't have calipers, you can measure the circumference and divide by pi to get the diameter.