using american infantry and making them German

You would have to modify the figure.  Your best bet would be to hack them.  Get the body you want and find arms and legs that go with them, so you dont have to mod everything. 

The uniforms are wildly different between the two, anyone that knows what there looking at will notice right away.

Tankluver wrote:

Is it possible to paint American soldiers as German will it e noticeable or would I need to make modifications to the figure?

Given how many more German figures there are than American ones, why would you need to? You'd have to sand/shave off every uniform detail, regardless of branch of sercice, and resculpt them in appropriate German styles. The uniforms of the two countries had nothing in common.

Typical German uniforms:

Typical US uniforms

Tankluver wrote:

Is it possible to paint American soldiers as German will it e noticeable or would I need to make modifications to the figure?

Is it possible? No... You'd just be painting American uniforms in the wrong colors. Will it be noticeable? Oh, yeah... Would you need to make modifications? Oh, yeah..

As AJ said, the two have NOTHING in common with the POSSIBLE exception of the German "Retreat boots" and the US boots w/leather leggings... Were you going to kitbash some figures, the lower legs might work on either one, and the head & arms... But that's where it ends.  The torso isn't even close...   

But just painting them? No way, no how..