My lathe (10 x 22 import) has a 1 3/4 in. x 8 tpi spindle. It came with a three jaw chuck and a 6 in. 4 jaw chuck seen here:

Note that the backplates have threaded holes to match with two locking devices that can be used to prevent the chuck from screwing off the spindle. It also came with a faceplate - not shown.
I also have a new 5 in. 4 jaw chuck that requires a backplate. This chuck is lighter and more useful for smaller jobs than the 6 in. chuck so I decided recently to get at making a back plate.
Spindle:

Last week I picked up a piece of ductile iron - 6 in. in diameter and about 1.5 in. thick. I also had on hand a pieces of 2.5 in. diameter hot rolled steel rod.

A couple of days ago I started to make a fake spindle with the threads and shoulder. The existing spindle was measured for diameter and length.
I cleaned off the outside of the hot rolled steel with a steel brush and got it running as true as I could in the 3 jaw chuck. The outboard end was faced and drilled with a centre drill so that a live centre would give it support.

(not sure if this is proper procedure but it seems to work).
The diameter of the shoulder is a bit greater than the threaded portion so I turned the full length to that diameter first. The threaded portion was then turned down just under 1.75 in. and a cut taken at the back of the threads with a cut off tool - to provide and "ending" on each threading pass.
A 60 deg. hss threading tool was set up and I threaded the majority of the thread depth under power. It is a bit tricky with this lathe as the slowest speed is about 150 rpm. Once it was nearing proper depth I first used a thread guage then used the faceplate and 4 jaw chuck to test the fit. Nearing the end my "threading mandrel" was installed and I completed the threading turning the spindle by hand.
At one point I thought I had the threads correct and dismantled my threading setup. The threading was found to be too shallow and I had to pick up the thread again. Being that the threads were 8 per inch it wasn't too difficult.
The final result can be seen below:

Today I installed the piece of ductile iron close to centre in the 6 in. four jaw and faced both sides. It had been cut on an industrial bandsaw but was still out some so it took a while and worked the lathe pretty hard.
Further to come although it may be a while as I will take my time with this.
John.