Is the car an original Cobra ?

Run another line of shielding gas to the underside of the welding area.

yes it is an original cobra. it has been raced since new in '64 so it was not treated nice. the aluminum is wrapped tight around the steel tubing so I am sure that it is pulling some heat and it seems to mess with the arc if the crack is a little wide which some are.
may try to get some shielding gas behind it but it is not like a tube you can close off so not sure how to contain it.

Most likely the aluminum is oxidized/corroded around the steel part, the aluminum acts as an anode for the steel.

Can you get a piece of aluminum behind to use a back up plate?

If your wanting the welds to look like bird **** then Im your guy.

i can't think off anyone that would choose an actual, original alu. body cobra to upgrade their welding skills on....make her beautiful!

Have seen a few guys use a shop vac to draw shielding gas around areas to be welded when there wasn't a good way to get behind or under it.
Maybe a "band aid" if nothing else helps.

2nd the thought of being quite a project to polish welding skills up on
Good luck with it all

hey, no guts, no glory!

Sent you a pm with name and number, He's your guy, one of those welds tin foil back together types.
great guy, he'll talk to ya, call him up