Unfortunately Foxtrot, Gargantua is pretty much right.
Gave it to you in a slightly different manner, but he’s right. You may have just been better off buying the two games. I was looking to do the same thing as you. I had Spring '42 and bought the global boards. I found the Italian pieces for a fairly good price, and can use older US pieces as China infantry, but that still leaves me needing France, ANZAC and mechs and tac bombers for every country.
I even thought about not using France and just moving the starting date back (forward?) until after the Battle of France was over, and consolidating the UK and ANZAC forces (probably should be anyway). Still, just buying the mechs and tacs for this revised game would put me at about the same cost as just buying the global game.
Another option you might want to consider is just playing the 1942 game on the 1940 board. There the hardest thing you have to figure out is converting the setup. Could still use the other countries if you want, or just the 5 main powers. You could also adapt any of the NOs, air/naval bases, convoy rules, etc. that you wanted.