Perhaps too late to weigh in on this now, it sounds like you have pulled the trigger already ...............but just in case.
I had 3:42's with a 700R and went to 3:73's ........... I find the drivability improved and did not affect mileage one way or the other. I found unless doing freeway type speeds (70 and up), it tended to lug in O.D. and I would find myself knocking it down a gear. Now I find it in the sweet spot more often in OD at normal hwy speeds (50 and up)...........
I based my decision on my 55 Ponch, it had a a T-5, 5 speed with a 3:70 set in it. I found it to be a perfect combo, lots of pull in the lower gears, would bury the needle in OD. and get around 19 MPG if I drove sensibly (which was seldom the case)
You will have a final gear ratio of 2.24 with 3:50's or 2.38 with 3:73's, little more than 5% difference, (give or take a little, depending upon tire size) those both are still very long legs, the only reason I didn't go with 4:11's is the 700R has a really deep first gear 3.06 (I don't think it would spend much time in that gear) You on the other hand would be able to get away with it, beings how the TKO has a 2.87 first gear and .64 OD compared to the 700R's .70 OD ............ 4:11's would still yield a 2.63 final drive, almost identical to my 3:73's at 2.61
Let's think about this a moment, lots cars in the 60's came with 3:73's in 4 spd cars with no O/D and auto's with 3.08's for mileage, you will have better than that with the TKO/4:11 combo and even more fun down stairs than the old Muscle cars with their 2.20 or so 1st gears (You will have the best of both worlds, that's what the TKO's are made for) Yah, It will still be drivable with the 3:50's but it will suck big time in O/D unless your on the freeway doing 80 MPH all day long....... Trust me, 3:50's are on the low side for a .64 O/D, they may even be a detriment to mileage by not putting your motor in the sweet spot at moderate hwy spds in O/D......... lugging is not good.
Just asking here ..........Is mileage really that big of a deal here, is this a daily driver? If it's not........... I wouldn't let mileage alone be a deciding factor in gear choice, driveability, sweet spot and the fun factor would be my main focus, besides mileage obviousy wasn't a factor in the engine choice (450 hp 383's are not generally reknowned for their fuel conservation)
I suppose the question is , where do you do most of your driving, If it's Hwy I'd do the 3:73 ................around town and back country roads 4:11's but that's me !!!
YMMV but don't let that stop you from having a blast
That's my 2 cents for what it's worth ............
Cheers, Judd
