I have always used Marine Tex. So, I'm not able to address the JBWeld part of your question.
I'm also not familiar with your action. But, when dealing with non-flat areas, I usually do some variation of the following...
(1) wallow out the area underneath with the Dremel
(2) apply tape and lots of release agent to protect finished surfaces and to avoid gluing the action to the stock
(3) build dam(s) as needed to force pooling in the area that needs bedding and plug extraneous orifices using modelling clay
(4) use alcohol to remove dirt, grease, release agent from bedding areas that need to adhere
(5) fill in the area gradually using Marine Tex and tamping with toothpicks and splintered popsicle sticks to remove air pockets
(6) sufficiently overfill the area and sink the action into it
(7) remove overflow with popsicle sticks, heavy duty paper towels, alcohol
(8) allow to set and then cleanup and inspect