How to maintain similar weight transfer after altering weight distribution?

[Zico]

Ok, Im no chassis expert so Im hoping some of you may be able to offer some suggestions as to how the changes we have made are gonna affect the handling, and what direction we should go to try and maintain or achieve as similar to the original weight transfer ratios as possible.

We have 2 near identicle Mk2 escort Grp4 rally cars, the original and the new improved, and more powerful one which we are in the process of building.
Both have a 5 linked Atlas axle, the new one is fully floating while the old one has half shafts and because they are forrest cars we will be keeping the leaf spring set-up.
The new car has a larger diff tunnel for improved suspension travel should we choose to use softer rear springs.

With the new car we have moved the engine back 15 cm to the rear to a Front/mid position and will likely fit a carbon/kevlar sumpguard to remove some weight from this area and to improve the PMI, we also aim to move the fuel tank in the new car to a more central position than where it is right now (above the rear axle) Whilst improving PMI.. This is gonna alter the weight distribution and affect the whole balance of the car so Id really like to try and work out how it will effect everything.

My initial thoughts are that we will be able/have to to use softer springs on both front and rear but I need some formulas to work out how much softer, I'd also like to know about the other side effects this is gonna create on the dynamics of the car.

Excuse the amateurish attempt at trying to explain the situation.. All info and pointers gratefully recieved.

Cheers..