since when does Young's Modulus address fatigue
factors in typical mild steel - which was my question?

But you really asked two questions:

1. Is it strong enough? Which everyone here agrees is "no,"

and this one:

My thought is to replace the ARB with a 4130 heat treated version , but I
want to drive the car first to see if the bar rate is close to right.

which the Young's Modulus answer DOES address. This is
because the YM of mild and 4130 is the same, they just
fail at different points (the 4130 can take more load
before failing but under the same load both respond the
same). This means the behavior and handling with the
existing bar will be the same as with a 4130 replacement
of the same design.

--Glenn Lyford