How thick of metal for frame connectors

This is not a race car and I am running a /6 :glasses7:
All I want to do is one step at a time and this is my next step :coffee2:
I am buying new metal for my frame connectors and don't want to over do it.
I will be useing 1''X3'' , so my question is this, what is the thickness I should ask for ?? to be welded in my 66 Valiant frame.

Thank you for all your time and tec help :cheers:

doing it this way, THank you all very much