Jim,
You can try advancing the timming and see if it will help, but there can be so many other issues that can cause this problem. I would suspect that these models were never designed to be barn burners off the line. New wires and plugs can help If they were actually needed. Your engine has been slightly detuned to meet requirements during the late 70s.
What can be happening.
Yes, trans can be slipping, torque converter, fluid level?
Vacume advance may not be advancing correctly
Fuel metering or flow
Ck rear gear ratio - what RPMs at what speed in final gear, tall rear gear will not make a quick start
Distributor worn
Valves need adjustment
Cam worn
There is a whole host of issues that can be happening, but without going through them all, a step at a time you are only guessing
You are pushing a lot of weight in these autos with not a lot of torque to do that
Try power braking and see what happens
My SLC 3.5 L will not spin the tires off line even with power braking, just sits down and grunts, gets its rpms up and hits second and slams it hard, spins the tires a little and starts really pulling. I know that mine will keep up with a Supra Turbo up to 60-70MPH, actually be ahead of the turbo. 0-60 rating on a S. turbo is 4.1 to 7.1 seconds depending upon the year of the Supra.
My rear gear is the 4:02
Durrance Engineering told me that my model has already the distributor coming in full advance at about 3100 rpms, whereas the 450 are coming in around 3900-4100, way too late for performance. When I purchased my auto, it was OK, except that the Mercedes mech had set everything for a 450 SLC instead of a 350 that I have. Yes, it was somewhat of a slow dog.
Here's what I have done.
K&N filter - noticed improvement - $36.00 at most auto stores, actually one that fits a GM vehicle
Made a water injection system - made myself- noticed improvement, sometimes sucks too much water
Direct Hits - yes, expensive but big improvement, increased power, reduced emissions, and upped MPGS a lot
Took off the first muffler and just left the rear
Timming advance to 20 degree before - I am able to do this because of the up grades - factory is10-12 - you can not just advance timming this far and hope it will work, you have to do other things to allow this to work. Rule of thumb, advance timming to just before the engine pings going up a hill in final drive, do not let the engine ping. This is just trial and error. Set, test, set, test until you are just under the ping point.

Just checked your factory timming is TDC, for every degree of timming you are able to add you will pick up 5 HP, TDC is not a good place to be for MPG or performance.

I am not an expert on these autos, just a old Pontiac motor head that held a E-stock record for 2 years in the late 70s to early 80s with a personally hand built Pontiac. All that I can relay is what I have personally expericenced. Durrance Engineering can provide a dist. that is recurved and requires no points for around $300 plus. If you have no other internal problems or fuel issues, trans is good, fuel delivery is good, dist and advance is working good, then it looks as if recurving and advancing timming should do what you want.

I noticed that you are in Grey GA, I am in Woodstock, GA. If you need some help, just look me up. No one has ever been able to figure out why, 2 autos side by side on the production line and one will come out great and one will not be so great. Age old auto questions. In searching I have not come across anyone that has paid a lot of attention to the real perfomance of the SL and SLC lines, have not found anyone that really put their head into these and really made improvements. Except maybe Anthony in England. He knows a lot.

Best of luck,
Tim