Tuning ducted fan engines is almost a lost art. During it's heyday, the OS77 was definitely a workhorse. I still run a bunch of 91s but, obviously, none of the "numbers" in my notebook will help your 77. With one exception, all of them have been massaged by John Ackerman of JFA Engines [see link <http://rcboat.com/antjfa.htm>] and I'm pretty sure he could get you in the ballpark with some good tips if you contact him.
Fwiw, the 91 package will give you more static thrust for sure and that would be a good thing on takeoff or if needed for a blown approach and you need a go 'round.
I would NOT recommend configuring in-flight mixture control in the middle of the setup you describe - it needs to be on a dedicated channel so you can use it for what it was intended for: IN-flight mixture control. The difference in cost of a good 8- or 9-channel radio (compared to a 7) it not that much; certainly less than the cost of dorking up your jet if you can't get out of a lean run and deadstick... the F-86 is not exactly a glider.

Mike