Ronald lets make sure I have this right.

The ride height in the rear is lower. You have 155mm wide tires on the back but a welded diff for traction??

Also the tall rear tires (80 series) are going to have a softer / more flex in the side wall.............read softer rear suspension.

Lower rear ride height means weight transfered off the front tires

So you have the triple whammy going on: softer rear suspension / Rearward weight bias / Welded diff.................not surprised your fighting understeer.

I would ask what drove you to this set-up: did you fit the wide fronts after welding the diff to combat understeer? Then Skinny Rear tires in a further effort to combat the understeer?

If your car isn't belting out more than 85-90 Flywheel horsepower ditch the welded diff run an open diff till you can source an LSD.

I'll share with you the budget set up we started with: 185/60-13 Yokohama A008R tires, 7/8th (21mm) front bar, (15mm) 5/8 inch rear bar , cut down 240Z springs front with lower blocks rear dropping the whole car 50mm (2 inches), Cheap gas struts front and KYB shocks rear. The motor was near stock the first year we raced it but the car handled very well.............not as good as the current proper suspension but good enough.

I'd be willing to be getting the ride height equal front to back and four of the same size tires (even with the welded diff) would result in better lap times then the current set up. Don't know what kind of circuit your racing on but I also suspect with an open diff what you gain in corner speed would more than make up for the bite the welded diff offers in slow corners.

Tom