Do you have a fuel pressure gauge you can watch on the gear change? On some really hard leaving cars you can stall the fuel enough to cause a drop in pressure with a rear mounted pump and it might just show up now because you are leaving harder and pulling the fuel level down at the top of first gear.
If your float level is not set right to begin with it might help to raise it. Does the carb have an extension on the rear bowl and jet extensions on the jets. It sounds like you just reached the threshold of the fuel system if you weren't having problems and now you do. Just some things to look at.
Curtis