Hi Paul,
If the nose is going to get a lot of use, I'd use something hardenable, but beware dimensional changes during heat-treatment! You may need to finish the taper by grinding once hardened and tempered, so leave a thou" or so allowance and hope nothing distorts too much.
Grinding to size with a toolpost grinder will give you the certainty that all's concentric too, very light passes (a few tenths of a thou") until there's a couple if thou" gap between original spindle face and the flat back on the D1 nose, the mounting bolts' tension should then close the gap and give the right interference fit.on the A1 spindle to eliminate any problems with rigidity etc.
Then repeat and grind the taper and flat on your new D1 nose, Robert's your parent's sibling!