One more question before I power up my lathe. Do you think 6061 will be good for this insert? Or should I use steel.

I don't like mixing brass with alu because of differnet temperature expanson and corrosion (electical). 6061 might be strong enough to hold the glow plug's thread. My first gutted spark plug had aluminium insert for glow plug. and It still holds strong. Actually it holds to the gutted spark plug only by press fit from the inside (ther is a little flange from the inside) and I never had a problem. So it might work.