my lemans has a happy mildly worked 350 with around 320horses which is attached to a rebuilt turbo 350 with a 2400stall and shift kit. Im very happy with this setup right now but i would like to stick a better rear in the car. right now i have a 10bolt posi with 2.56 gears. Great for the highway! but not so much for showing off the power of the engine. everyone tells me 3.73s are the way to go but i dont want to lose my highway driving completely. will 3.73s kill my highway driving or will i still be able to cruise at 65 70?  

I would recomend a 3.23-3.42 rear gear.  

the shop down the road from me (who built my transmission) tells me 3.73s are the only way ill really feel it. Says everything else is a joke.... i was leaning towards 3.23s when i started working the car but my father and others started to convince me to go with a steeper ratio, 3.55s were stuck in my head for a while now... but will 3.23s or 3.42s give me that pushed back in the seat feeling? (my car only has 2.56s right now and shes not exactly slow off the line 0-60 right now is a touch shy of 7seconds.) and im lookin to hit 60 in about 5 and a half with 3.42s will i feel a jump like that or no?  

What is the height of the tire you are using. with a 3.73 gear ratio you will see a big decrease in gas mileage.  

a couple sizes bigger than stock....255/70R15s
but in a few months ill be putting stock tires back on because i like the stock stance much better than this slightly raked style i have going on. a taller tire should make the car slower off the line and would cause the engine to rev a little lower at a higher speed since its a larger rotating mass correct?  

Correct, With the tires you have and 3.42 gears your speed will be 75@ 3000rpm, with 3.55 gears 72@ 3000 and with 3.73 gears 69@ 3000.I think the 3.73 gear will be to low for the highway.  

thank you very much i think i will be going with the 3.55s sounds like ill be very happy with them to me! thanks again!  

Your well come.  

I have 3.55s with a 26" tall tire and in hi 1 to 1 and conveter locked-up it turns 2750 at 60. In OD it drops to 2050.