like the title says...can i tie in a Moly 10 pt roll cage with mild steel sub frame connectors? would it be better to cut out the old MS SFC's and replace with CM ones?

 

I was wanting to do 4130 cm subframe connectors and Steve Spohn advised me that to properly install them, I would need to have them tig welded since my car is made of regular steel.
I would assume that your situation would be the same.
However, I've heard that during WWII cm was used in some aircraft and that one of the other methods of welding were used for that, but I don't remember which.

 

Absolutely. TIG is the only method allowed by the sanctioning bodies for welding CM. Use a mild steel rod - I prefer ER70S-2. No post heat-treating required. I'd keep the MS subframe connectors. It'll be a real pain to cut them out, and there is little to be gained (weight wise) by replacing them with CM steel ones. FWIW, I have a full MS 2x3 frame in my car with a full CM cage.