Thanks to both for the info. 2 inch per foot = 1:6 so that all lines up. Interesting I spoke to a guy at Speedway and he thought all the late-model metric stuff was 7 deg taper, which is closer to what I measured... but I will take Howe's word over it any day as they make 'em
I am already looking at Howe adjustable ball joints so I can vary the roll center and didn't even think to talk to them directly about Corvette stuff so I will contact them.
I think what I am going to do at this point is machine up an insert to replace the ball joint in the
spindle with the taper, then use a ball joint in the upper arm which is more of a "normal" setup, simply flipping the joint over and have it point up into the spindle v. down like say an old Camaro/Chevelle setup. Having the joint in the spindle is odd so I was going to have to make a thread-in tapered item to screw into the IMCA uppers I am looking to use anyway. This way I get the exact ball joint I want in the IMCA uppers and the exact reamer dimensions for the top side of the spindle.
The joys of trying to do custom stuff but maintain as much "off the shelf" stuff as possible for ease of maintenance - I end up with two ball joint holes at the top and no tapered side to bolt into
edit - sorry, forgot this part from the link above
"One unique feature of theGMR uprights is they have matched the stock lower ball joint taper perfectly using a custom built reamer. They will not disclose the exact taper but we can assure it is not either of the standard 7 or 10 degree tapers.Their uprights matches the stock taper perfectly to ensure quality fitment with the stock GMR components"
I can say that the math on my upper didn't seem to match either exactly so maybe that is what they have found as well, but maybe Howe determined it to be "close enough" to 10deg that is works? This is why I am thinking I will just make a plug for the upper side of the spindle and ream it to an exact known 1:6 Howe ball joint for the IMCA upper.