Will this 8 1/4" work in place of my 7 1/4"?

What difference does it make how much it costs? You need to have it done. No way will we have that info, since we all live in different areas and cost is different everywhere.

"I" could get it done for probably around 100 bucks parts and labor. Your results may vary.

But then "I" would do it myself......as you eluded to.

All that's needed is a good straight cut. That's the key to it all right there. I have done probably a hundred through the years and got no vibration whatsoever.

I use my woodworking miter saw with a metal cutting blade. Works like a charm. Make sure the yokes are timed, like you said. I weld in 2 opposite places first. Then stop and make sure the yoke is good and still snug against the shaft. Then weld in two more 90* opposing points and finally finish all the way around welding about an inch at a time so as not to over heat the shaft.

I am sure there will be professionals who will scoff at the method, but it works.

At least it does for me.