gear in 240 noooooob question

[240Ferg]

Read First then flame me if you must
I drive a 91 240 se (open diff) I dont want to drift, drag or anything else just drive, alot. 100+ miles per day 99.9 miles of that on the interstate at 80mph and I want to lower crusing rpm add fuel milage. I know other diff's will fit j30, skyline, 300, but the gears are wrong for what I want to do 4.30-3.69 I know the 92 300tt has a gear of 3.69 and that is the best I have found. Looking to get to 3.40+/- I have spent alot of time calling, searching, reading but most posts are about vlsd's and what cars have them. Have I missed something or is there no aftermarket ring & pinion out there? I have thought of tranny swap but 5th gear= .79 underdrive in mine and 6th is the same in most others I have found.
Any help would be great. I understand not all of you will agree with killing the low end but It is my car. But I will be looking for that 4.30 clutch type diff for the street, strip show 240 soon. Thanks