Thanks Davo. The stationary jaws do bolt on from the back. I tapped the jaw holes, ran the bolts in, and then milled a skim off the whole jaw face so the bolt ends are flush.
The moving jaws have countersunk flat head bolts that pass into tapped holes in the moving block.
All jaw faces were milled at one go, in place, with the vise bolted square to the mill bed. I should probably put some dowels in.
The JB epoxy putty I used is good stuff for filling casting imperfections, but it isn't what we refer to as JB Weld usually, though made by the same company. The putty is not described as "structural". But it's a lot better than Bondo.
Regular JB Weld is a two part metal filled epoxy that comes in tubes, like most epoxies. It has a slow (overnight) cure. It is extremely tough stuff, a structural epoxy with a high temperature rating -- 600F, believe. By comparison most common epoxies are softened at about 200F.
JB Weld is legendary (or infamous) in the old tractor world, depending on whether you are a farmer or a museum quality restorer!
Case in point: I have a 1951 John Deere Model M tractor that is worked hard. When I got it I noticed that the front axle pivot pin had worn an egg shaped hole in the front casting. There was no bushing. Just a hole in the casting. Probably worn through decades before. I could not replace the large frame casting, but replaced the rubber supported bushings in the floating axle support tube behind it and filled in the casting ellipse with JB Weld and drilled it circular to correct dimensions.
The pin is not supposed to rotate in that hole, although it had in the past because the other bushings were shot. So there is no frictional wear normally. But the full weight of the tractor front end is on it, and it gets hammered. That repair has lasted ten years since then with near daily use on rough terrain -- log hauling, etc. -- with no sign of wear of cracking.
So anyway, JB Weld is a good product for cast iron repair where other methods are not available or practical. In my experience, anyway.