Gang, I have a fairly stock "G" block 400 (71k miles) with the following modifications:
JM 20-22 cam
Edelbrock Performer Intake
QFT 680 carb
1800-2200 Stall converter
Comp Cams roller tip 1.6 rockers with hardened push rods
Hooker Headers with 2.5" exhaust
HEI ignition with MSD 6A box

The stock 12 bolt was rebuilt to include an Auburn positraction unit with the stock 3.07 gears.
At the track, my car ran a best of 9.46 & 73mph in the 1/8th mile.

I would like to get the car a bit quicker, what can I expect going to a 3.42, 3.55, or 3.73 rear?
Tires are 225/60/14s and are about 26" tall.

 

Define "a bit quicker". Any of the gear ratio's could improve your E.T. At some point, tire spin will need to be dealt with.

 

How much quicker will my et be with those ratios?

 

Be aware that if you have an O-Type rear (12 bolt cover, 10 bolt ring gear) there aren't many gear ratios available. Aftermarket there are 3.42, 3.90, and 4.10 gear sets from Richmond Gear (https://www.richmondgear.com/wp-cont...n1.pdf#page=32).

If you have a Chevy 12 bolt, then the ratios you mentioned are possibilities.