Ah, I understand. I can't help you with the former, but would humbly suggest running them linked wherever they are and fine tuning L/R Balance in post, but I know you're more of a "get it right the first time" kind of taper. Hopefully someone else with a 7XX can help.
With the latter, I'd advise running the 744 at 0 or whatever is closest to "unity gain" (which I know doesn't actually exist, but is still useful as a heuristic model) and using the gain knobs on the external pre entirely. It seems to me that one would use the SD 302 in order to get some transformer distortion "warmth," so why not push it. It isn't like a digital stage where a hard 0 is clipping....