A Powerglide (did your GTO really have 2-speed Powerglide installed?) weighs a scant 100 pounds without a converter, a TH400 is about 145 pounds. The 4L70E outweighs the TH400 by about 25 pounds so no weight was saved with the trans swap.
Factory air conditioning on these cars adds about 100 to 125 pounds, so that's a big reduction right there if you're removed all of it.
I see some lowering springs in your future.
On my '64 GTO I went for some stock '67 Firebird V8 front springs to lower the car a couple of inches.
Some stock six-cylinder springs could work, but they may not be a great performance/handling spring. The '68 Lemans and Tempest came standard with a 250-inch overhead cam six under the hood, the six weighs around 440 pounds and that's within about 20 pounds of the LS3 with the six probably being the heavier of the two.
If you do order some lowering springs perhaps some that are made for a Chevelle with a smallblock would do the trick.
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