Probably could use some body filler for something like this. Might even form some foil to the lathe bed first. You basically need to model your fixture after the tailpost base.
Only complication here is you want to be able to locate it back and forth or up and down so you can vary the distance radially from the spindle, in which your drill will be mounted. If you cast a base to follow the ways, you are not going to be able to move it across the ways. You could probably get where you need to be by shimming the height. So you would have, from bottom to top:1 - Base molded to shape of bed and the size of the indexer base. 2 - shims vary the height 3 - indexer.
This sandwich would likely require a plate under the bed, so use the one for the tailpost, since you don't need the TP for this op. You will have to have a through-hole or two to hold all this together and in place. It might be easiest to thread the bottom of the indexer for a single bolt coming up from the bottom. You'll have to figure out that from the bed, indexer, and lower clamp that you are working with.
Taking an entirely different tack, you might be ahead to build your own fixture, based on a small angle plate in place of the toolpost, and using a 28-tooth (or 21, or 35, 42 etc) gear to index, as someone else pointed out. The only fabrication would be an arbor to accomodate your wheel on one side of the angle plate, and the gear on the other side. You just need something to engage the gear to lock it in place. Once you build this once, I'm sure you will use it again for other parts of your project. Note that this could also double as a simple gear hobber. Google on Gear hobber or gear cutting to get some ideas on this.
- - Rex Burkheimer Fort Worth TX