I modified my normal gunsight (my take on the world renowned death-dot) so that the bullets go through the center of it on the F4F-4 and FM-2. I know that most of you don't use the single dot type of gunsight, but this may be a good starting point for you to make your own or adapt your normal sight for these planes. I tested it offline in both planes, at 500 convergence and 250 and all the bullets hit the dot.
Keep in mind that HTC may be fixing this down the road, so I don't know how useful you will find it. Anyways, hope it helps.