So, the lumps are finally hard soldered/brazed in place. This, however, took 3 tries to get a good fit! First the lumps should be a good 'snug' fit in the monoblock. I had too loose a fit. The lumps 'settled' downward and the assembly wouldn't close on the action. There must be a couple thousands clearance between the action flats and the barrel flats.
Next, my milled slot was too wide. On the 2nd attempt, the rear lump shifted about .001 sideways, not allowing the rib extension to go in.
My advice here is to make sure you have a tight fit before you solder. I would have been wise to have taken a center punch and raised some 'points' in the bottom of the monoblock. I tried some brass shims........they obviously melted! You are getting almost red heat to melt the solder. I think I used 40/45% silver (I've had it a long time).
The top pic shows how the lumps should fit the action when you start. No fore and aft movement, extractor in place, locking lug engaged. I think the best advise here is to keep the action totally stripped except for the hinge pin and the locking lug in hand. The crossbolt hole will be the very last thing done!
As for heat: I use an ice melting torch, cheap, on eBay, comes with 3 torches. I use the mid sized. Cheaper than a Siefert for most things. The torch is a bit too long but can be held.