Low gear in the ST300 is pretty close to 2nd in a Th350/400, so you will be able to go a bit faster in low than you can in 2st with the 3 speeds. The 700R4 and 2004R have even lower 1st gear ratios, the 700 has a 3.06 where the 350/400 have 2.52/2.48 first gears respectively.
High gear in the two speeds and 3 speeds is always 1:1, in the 4 speed autos 3rd is 1:1 so mph will be pretty much the same in those gears provided the converter isnt locked or slipping excessively.
Fun with numbers. a 2.41 gear with a .62 OD in the 2004R, a set of 28" tall tires, and 5000 rpm = 278 mph..
Stupid fast huh? At 1500 rpm you are traveling around 83mph with the same setup.
You would need at least 5700 rpm and 28" tires to go 145 mph with 3.23 gears, provided there is no slip.