I've been welding 4130 race care chassis for nearly 25 years now and you should tack and weld it with TIG method. You can tack with ER70S6 if you must but it just causes you to have that tack in the middle of the weld that looks bad. For TIG use either ER70S2 or S6... Most people still swear by S2 but the mill spec will allow S6 to be used anywhere S2 is called for and if it's good enough for mill spec I think the race car world can allow it.. The original wire was Oxweld 65 if my memory serves me correct.. Anyway I like S6 a lot better because the higher silicon makes everything flow out a little nicer. Be sure and use a foot control and don't over heat the tubing because it makes it brittle. Also 1/16 wire is the choice size for this.

I hope this helps you get off on the right foot.

CK makes some very nice torch mounted controls that aren't all that expensive..I know my next rig will be a CK because they have a 300 amp torch that isn't any larger than a WP20 weldcraft and takes all the same parts as a WP20..