Boring-head lathe tailstock adapter (MT3)

I bought a boring head that takes a threaded 1 1/2" - 18 tpi adapter. I wanted a MT3 adapter so I could put it in the lathe tailstock. Because I'm "frugal" I opted to make it myself. 

• I used a 1" mild steel piece for the taper and a 2nd piece welded on for the large threaded portion. I failed to take a picture at that point, but I turned an alignment nub on the end of the rod that matched center drill on ~2 diameter x 2" larger rod. 
• Using that alignment, I welded them together.

This is where the pictures start, 2 rods welded together and roughed out:

1.5-18" threads cut:

The weld looks terrible. Either I was having shielding gas problems, or the 2" diameter stock is a leaded free machining stock, or ????

The final product:

I turned the taper by setting the compound angle to match an existing MT3 part and only advancing via the compound feed.