With your current ratios, shifting from 5th to 6th is causes a drop in hp, most likely because the tall gearing that you are using is causing engine RPM to fall to a low value.
You wan to make it so that when you shift into 6th, the RPM's are high. This will allow good acceleration. What exactly determines "high RPM" will be found in the car's powerband. Ideally, you want to make it so that the 5 to 6 shift happens exactly at the RPM where 5th gear has the engine turning at a RPM with the same hp that the RPM in 6th gear will produce. This will lead to equal acceleration on both sides of the gear shift.
Besides just changing 6th, I would suggest lowering the gear ratios of 1-5 if possible. Especially 1. GT5 has terrible gear setting options because you can barely move the ratios around, but you might be able to make some small improvements.