While there are @ 200 degrees difference its still a risk. In the past I've used localized flame heat on the HAZ working in from the tube side then reduced T by reducing temp of flame and moving flame away and down tube from HAZ. This way high temp area is kept well away from any brazed joint which seldom get above 250f. I certainly would not want to subject whole monobloc to 1450-1500 F.

Hugh Lomas,
H.G.Lomas Gunmakers Inc.
920 876 3745