It depends on whether the top is thick enough in the area you want the slot. I'm assuming it's cast iron. Typical thickness could be around 1/2" with additional ribbing. A 3/4" x 3/8" slot won't leave much meat on the underside and may seriously weaken the tabletop such that it could break.
If you feel you can get away with machining, it should be a straightforward job for a general machine shop in your area.
In a similar vein I had some mods done to my cast iron router table with no problems. It cost me £30. The chap had to write a piece of code to do circular interpolation to end-mill a 180mm dia recess, so I reckon that was good value.
cheers,
Ike