Open 3.07 to posi series questions.

[firefighterchevy]

Well, the internet is full of great advice...just sometimes confusing and sometimes incorrect. I hope to gain some simple answers and knowledge from you folks who know this stuff best. So, this is my plan: I have a 69 C10 that currently has an open 3.07 rear axle. I have a 3.73 posi I bought that came from a 69 C10 also.
First off, I have nothing against 3.73 gears, in fact, I have them in all my other pickups and Suburban. I just don't want them in this pickup, as I would like to keep some MPG and top end speed at lower RPM's.
As far as I can tell by reading all over, the 3.07 is a Series 3 setup. I have read various places conflicting info on the 3.73, as to it may be a Series 3 or a Series 4.
What I want to do is stay with 3.07 gears, but make it posi. Can I take the 3.73 carrier and put the 3.07 gears on it and install it in my 3.07 axle housing? Am I going to need a gear shim if I can? Will this even fit/work? I am already aware of setting up the bearings with the swap, if I can. Can some of you folks point me in the right direction? Let me know if I forgot some info that may be needed, and I did a search and couldn't find a thread like this here. Thanks.