Lower ratios are Available but they will raise the RPM in the lower 2 gears. Even with the lower gears installed high gear will still be direct drive with no difference in RPM.
TH400 2.75 Low Gear Planetary - TCI® Auto
TH400 (2.75 1ST, 1.57 2ND) LOW GEAR PLANETARY Retails $768.75
I'm sure that this gear set would be cheaper though their authorized sales outlets.
If you would happen to be doing a rebuild these gears wouldn't take a extra min to install. But a trans has to come Completely apart to install these gears.
Stink is probably right on you RPM difference now . 99% chance the torque converter is just a bit "tighter" A hgiher stall speed will result in higher trans fluid temps and worse MPG. Something i suspece you 83 GM van needs no part of.