If you are welding to D1.1 or ASME, the answer is "no", you are only qualified for the material you joined.
AISI does not include published mechanical properties for any of their base metals with an assigned AISI numbers. The reason is pretty simple, the mechanical properties are dependent on the chemistry, manufacturing process, and state of heat treatment. The AISI material specifications only warrants the chemistry, they do not include the state of heat treatment or manufacturing process (think hot rolled versus cold rolled). That being the case, how do you know if you passed the tensile tests or not? That's why most welding standards do not list AISI materials.
Why are they using a high strength low alloy steel that is heat treatable to increase the tensile strength to more than 100ksi and then weld it with 70ksi filler metal? Strange, very strange. Do they plan of doing a heat treatment to the finished weld? Stress relief, normalize, anneal, quenched, quenched and tempered? What is the state of heat treatment when you weld the coupon? They said they will accept 70ksi for the UTS, but what are they expect to do with the weldment after they receive it?
A change in the carbon content of 0.1% is significant in my world.
You'll have to review your welding standard very carefully.
Good luck my friend.
Al