Mark,
I have no idea if the 409 is compatible with what your going to use.
I know for sure it is compatible with R-12.

The head temps aren't an issue here, it's  the head pressures that's
important, and the 409 runs at the same pressures as the 12.

I'd have the shop of your choice dump 12 in your van, and then if it
runs low then use the 409.

I think this is the way to go.

Later,

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Terry--
74 Campmobile- (Clementine)
85 GL- (Delilah)
86 BMW 325 ES- (Eva)