What I have is essentially a clutch setup, with a stainless steel hub that has the bolt hole pattern of my flexplate for my automatic (aka clutch plate) and then a taper locking piece that has the 7/8 inch bore for my shaft, which is then bolted into my stainless steel hub. Here's a picture of it:
S5001540.JPG (image)
Now since I did an automatic transmission, my design was the same as a clutch setup. If I were to do a conversion like yours and opted not to keep the clutch, I would do what you plan to do with welding the two female spline pieces together, but then I would place them into a tube and weld them again to the tube for maximum strength.
And here's the link to my conversion if you're interested:
http://civicity.blogspot.com