In the RCM thread "Making a micrometer stop for a Clausing lathe" in July 2012, I talked about getting a pair of 35-degree (half angle) conical mill bits (V-203 from Conical Tool Co). They came a while ago, and I just today made my first trial cut on a block of scrap aluminum.
This works a treat. My first trial grooved block sits quite solidly and stably on the 70-degree (included angle) V-way ridge of my Clausing 5914 lathe. It took me all of ten minutes to make the trial block.
I roughed a 5/16" slot first, before cutting the V groove.
As for the Conical V-203, it seems very well made, and the fit of the shank into the Waldon holder is almost airtight. It certainly cuts very well.
Joe Gwinn