The only thing I can think of is a R8 spindle from a Sieg X2.
Not sure how strong it would be though on steel.
You would have to make your own head casting to seat the bearings like part 114 in the diagram link.
The top part of the shaft where a pulley would go is 28mm diameter I believe.
Spindle:
https://littlemachineshop.com/produc...ProductID=1407
I've seen cheaper. This is just for quickness.
Bearings:
https://www.arceurotrade.co.uk/Catal...indle-Bearings
Diagram:
https://www.arceurotrade.co.uk/machi...-List-2012.pdf
Or scrap R8 altogether and get one of those stupidly expensive BT40 ATC spindles from China.