I just had an overdrive trans put in my car cause it had 3.73 gears with 225-60-15 tires.. it was NOT fun to drive on the highway. but around town stop light to stop light it was cool.. I wanted to be able to drive the car not just around town so went with an over drive trans.. if you are planning on keeping the 4 speed manual I would say 3.42 gears or 3.23 or even 3.08 gears.. with those gears if you had to get on the freeway it would Not be winding up so bad at least.. just remember the lower the number gear set the slower the car will be off the line..

 

Maybe this is a little late but I am running a 383 - 350 turbo with 3.31 gears. I had an eaton posi trac installed running 255/65/15 and I have cruised the highway at 75-80 at about 3,000 rpm. I did put an aftermarket aluminum radiator with a Mr. Gasket 160 degree thermostat and I never see over 165-170 degrees at that speed in the summer. Seems like my car loves driving that speed either that or I have a heavy foot lol. But for your car make sure and buy the GM posi additive from GM. I had issues with clutch noise with every brand including royal purple. After I added the small bottle no issues. Just FYI. Good luck nice car!

 

I took out the 308 put in 373 in my cars works great enjoy

 

No not too late. It looks like right now I'm starting with 350 with 700R , 3.08 gears and 255/60/15. I know it wont jump off the line with this but...it won.t eat all my money too on hywy. I will try to keep Rs under 3k...if possible ! Thanks