Well it is moving forward, a Customs form is a start.
If the Customs form has a chassis number on it, and that matches the number on the jeep ( should have plate on the chassis if an MB ) then it is possible.
On the other hand, if he is rebuilding TWO jeeps, using copies of the same Customs form, and he finishes his first, it is excrement river with no propulsion system.
It is still worthy of consideration, but frankly, I'd only look at it if I was getting BOTH chassis. The other one was for parts and broken up as 'yours' was the better chassis - then let him throw in the other chassis in the deal.
If he brought in two, but only one chassis number, potential hazard. I brought in two Carryalls on one C386 form and had no problem with BOTH chassis numbers appearing on the form for that shipment, duty paid, etc, etc