I'm a retired aircraft machine shop owner. Your application doesn't require AQ specs so, I would suggest the following:
I assuming that they will have to HTed? You don't need 1 5/8 blanks, 1.5 is plenty. You will get 92 pieces to a 12 ft bar @ 1.50"
1. Use 3/4 Hex, 1144 steel, It's easy to machine and as strong and hard as 1045. You don't need 4140.
2. Change counter bore to 7/8" or turn the lower part to 13/16
3. Call the local job shops, in your area and get multiple quotes.
4. Call the heat treater's in your area and get a quote for that.