I just used plumbing solder for the AK.45 barrel blocks. Since there was essentially no stress on them, the low temp solder works fine. I figure if it was good enough for the Mauser, it's good enough for me.
I used a Chinese gas block on mine because the Chicom barrel has a diameter of 0.630" at that point. I enlarged the hole in the gas block with a reamer to allow me to keep the barrel wall as thick as possible. The barrel O.D. on my .45 coversion is 0.670" at the gas block, so the block I.D. is around 0.680" It's a slip fit to allow for soldering.
This is a view of the modified Chicom block as installed on my barrel
The wall may not look that thin, but keep in mind that no gas goes through the block to operate the rifle. My barrel wall thickness is 0.106" at the gas block. This is perfectly acceptable for .45ACP, but there's NO way I would trust .45-70 with a barrel that thin.
This pic shows the location of the pilot hole for the retaining pin. Take note of the position of the barrel O.D. compared to the center of the pilot hole. If you drilled the pin hole as is shown, you would DEFINITELY bulge the barrel if a press-fit pin was installed.
The land on the bottom of the block isn't thick enough (after reaming) to allow for repositioning the pin holes.
I haven't been keeping up on too many projects lately, so I don't know if you've already got your barrel blank selected or machined. If you haven't turned it yet, consider using an RPK gas block. As a matter of fact, I would recommend RPK barrel blocks for everything on your conversion.
With an RPK gas block, you'll have a much thicker barrel wall. It'll save you a TON of work, too. Instead of making sleeves and welding, just turn the barrel to the needed diameter. There will be plenty of barrel thickness for the retaining pins as well, except the front sight block. If you try to drill and pin the FSB, you will bulge the barrel with press-fit pins. Just solder the FSB and call it good.
As I recall...
RPK: O. D. DIMENSIONS
Rear Sight Base 0.885"
Handguard Retainer 0.847"
Gas Block 0.758"
Front Sight Base 0.608"