I'll let one of the 750 experts answer that specifically, but, the 650 head is often paired to the 550/500 block as an upgrade. So overall, it might be a completely casting platform.
The SOHC and DOHC engines are quite different, so there's not a good chance of mix-match there.
If you have a good 650 head, seek a 550 motor lower end. The 650 is a bit of a performance upgrade and the 650 cam is often sought for its improvements also. If you're focused on a 750 frame and engine, you'll need to stick to that series. There are quite a few differences between upper and lower heads/motors within the 750 line too.
There is an Oz member, Terry, who has an enormous wealth of knowledge, and may also have some spares that are sellable. I know he recently offered something to another builder out that way. He also has enormous experience in shipping about the globe, so may have some insights in that as well. Perhaps he'll stop in and lend some additional guidance.
Retro knows his stuff very well too, and many others from your hemi who are active members here.
Cheers-
