Thanks everyone. I think I will be going with an 8" lathe, Grizzly GO768. I know it still has the same bore hole size as the 7", but the 6" chuck would be better suited for what I want to do.
I ran into this exact same problem with a 5 inch three jaw on the original Myford. It as a 7" swing machine and there was a size range that I could not hold in the 5" chuck due to this fouling. When that occurred I had to switch back to the 4" that came with the lathe. The reason I'd bought the 5" was because it was on sale and the chuck with the lathe didn't come with the outside jaws. And then I ran into this issue which you will too if you put a 6" on an 8" swing lathe.
You run into this with pretty much every lathe... My 12 inch chuck jaws can mash the 17 inch swing bed, and definitely trash the carriage. My 16 inch 4-jaw - I have to be damn careful.
I am new to this site, I am not new to lathes. I should have pointed that out at the beginning.
I had a Sheldon 13"x40", under mount motor 1953 year with quick change gear box.
I should never have gotten rid on this lathe but we were down sizing and I had it in my hanger about 20 minutes away so it was not easy using it when I needed it.
By the way if you wait and search you can find deals. I picked the Sheldon up for $800. I did have to change the motor out for a 240v single phase. But other than that the lathe needed nothing but a good cleaning. I was very accurate for as old as it was.
Jdaniel, if there is one thing I've found out from this forum it is that machinery prices vary wildly across North America. A 13x40 Sheldon in decent shape out here in my neck o' the woods would easily cost double that $800. And likely three times that if it's in truly nice shape. And it would not be available for more than a week once listed on Craigslist or some other method for local advertising.
Not that a Sheldon would show up more than once every 5 years or so though. It's a machinery wasteland out here for some reason. Even just south of the border in Washington state things are better. Not by a whole lot. But still better than up here North of the 49th.
There's no reason why you*should not take a little out, just remember to go just a little bigger than the workpiece. I recently bought a Toolmex (now Bison) Polish 6" four jaw chuck with absolutely no sign of ever being fitted to a lathe. It*was intended to screw directly onto a Myford lathe, 1 1/8" X 8 tpi, I believe. I bored it to 2" dia and drilled it to attach to a Smart & Brown faceplate. A fine chuck, the jaws are really tight, and as it was made of SG iron, there wasn't any cast iron dust when*it was machined.
I picked up the Sheldon in Minneapolis, so I did have to drive 800 miles to get it. I did spend about $275 to convert it to single phase and had to build my own mounts for the motor as I could not find a motor with the same frame for under $500.
When I did sell it 5 yrs later I got $2900 for the lathe. Not a bad turnaround.
I think I will try a 5" 4 jaw and bore it out. You said you used a face place as a back plate, how did this turn out.