I've got a couple T5s, but none in Volvos.
I'm guessing you'd be interested in a common T5 like the Mustang 4-cylinder one.

My Volvo has a Type-9 4-cylinder gearbox and 4.1 rear. Here's a quick chart:

Gear: M40: T5: Type-9:
1 3.13 3.97 3.65
2 1.99 2.34 1.97
3 1.36 1.46 1.37
4 1.00 1.00 1.00
5. X 0.79 0.82

The D-type overdrive is 0.756:1, and J is 0.797:1.

(Ratios: 1965 Volvo 122 M40; 1983-88 Mustang 2.3 T5; 1970s Pinto 2.0 Type-9)

So I chose the Type-9 because, well, it was already in the car. But for a car with moderate horsepower (under 125-130) it will drive like a stock Volvo gearbox with J-type overdrive. The Mustang 4-cylinder T5 'box will run out of RPM a bit faster on the way to 4th gear, then be about the same for overdrive.

I like the Type-9 in my '65 122. I have a Mustang with T5 and it's a fairly heavy, notchy shift (even with a performance shifter); the Type-9 is smoother. I can't compare to stock Volvo, but my mom's old '76 240DL had a fairly heavy shift, as I recall. But that was almost 20 years ago...

Not saying I dislike the T5--the Mustang would eat the Type-9 in about 5 minutes (especially the way I drive it!), but the ratios, smaller size and lighter weight (and smoother shifting) of the Type-9 seem better suited to an Amazon. An obnoxiously loud, tyre-smoking beast is allowed to have a rough gear change. ;)

I think a 122/221 with 4.56 rear and otherwise stock motor, 3.97 will be unpleasantly steep and you'll be shifting a lot at lower speeds. On a 4.10 rear, in hilly areas, it might be a bit easier to drive than stock. On flats, again, it will force you to shift to second pretty fast.

Some more T5 info:
http://www.moderndriveline.com/Technical_Bits/transmission_spec.htm

Hope that helped!
Ben