Ron,
I would not want to try and build anything much bigger on a 410 frame than a .22 Hornet. Your barrel stub walls are only goiung to be .078" thick and that is if you get everything just right. If I HAD to do this conversion, I would bevel all the joined areas front and back, down at a 45 degree angle and have it filled in with a micro TIG welder. That way if you do not get all the holding strength by shrink fitting the barrels to the monoblock, the weld will add enough to make sure you don't blow out a barrel.
I think that actually doubleriflejack has the right idea, make them a slip fit and solder them in with High Force 44. I'm still unconvinced that the monoblock will stand up to this much pressure. Are you planning to download this somewhat? Bob