My 3.42 rear that I took from a wreaked 00 TA was just WAY too loud. So I swaped in my old 3.23 rear. Thing is with my new 408, this has a massive 3rd & 4th. I like it! BUT I think a 347 ls1 h/c m6 with 4.11 is nice. I have them in my LT1 and love it. Thing is I've got such nice TQ and with 4.11s I don't think I'll ever hook so, I'm going with 3.73s. It's a thicker gear than 4.11s and my hold up in my 10bolt a bit longer. I'm not a racer and may have a couple passes a year but nothing more.
I assume the 3.73s won't kill my mpg as they are already bad with the 408.
when i had my 6spd i had my 402ci with 4.56's .....i wouldnt go less than 4.10's unless your doing top speed runs
all joking aside the difference in gas mileage is minimal go with 4.10's and who cares about gas when you have 500 plus hp