If the head on the 300 shows no signs of having overheated, I'd say there's as much reason to keep the head as to swap. As it is you haven't noted whether the 300 has an original or later head. Defnitely keep the 350 head as a spare.

If you keep the 300 head, transfer the combination gauge and afterglow temp sensor from the 350 head. And of course you'd keep the IP and injector lines. Jeff knows more about turbo compatibility but you should be able to make either turbo work by using the corresponding inlet pipe. Jeff, is it more complicated than that?

Sixto
87 300D