Actually yes and no. Muncies or Saginaws were the choices for manual shift cars in the 60's and 70's. Not all of them were muscle cars. Many of them were 283's, 302's, 327's, and 350's. And because of the 1:1 final drive ratios, you got your cruising rpm by having a tall rear end, oftentimes a 3.08, but sometimes a 3.36.
Usually, the small blocks, because of their lower bottom end torque, got the "wide ratio" M20 muncie, which had a 2.52:1 first gear ratio. That's not as good as later overdrive transmissions, but better than the 2.20 1st that appeared in the M21 "close ratio" and M22 "rock crusher" transmissions.
If the motor was a muscle motor and had the balls for the 2.20 1st gear, the close ratio was highly desired (especially for SCCA racing) because the gears were split evenly so that a minimum of torque was lost between shifts. But yeah, that 2.20 is a big first step!
I think the reason the Muncies go for a lot of money is because they are such good transmissions. Even the M20 and M21 could handle something like 460 lb-ft if your tires don't slip. What made the M22 so much stronger was the straighter cut gears that caused less side deflection. But that also made it noisier.
However, long story short, I think you'll be happier using a T5 with it's steep first gear on a straight 6 engine. You've really got to have a lot of low-end balls to make a 2.20 1st gear feel decent.
