Try asking the Datsun truck experts at http://ratsun.net. When I was a member there we sometimes got parts for Europe and Australian guys.

I suspect the brackets won't help you, as they are designed for the factory 620.L crossmember and insulators, which I don't think will fit a 620.J frame. Since brackets from any L16, L18 or L20B will bolt to your engine, I'd use a local bracket, there are some slight differences to shape so I'd make the custom crossmember to fit common brackets instead of the other way around. But ask on Ratsun, they might know.