wider rear axle,track width,suspension questions

[ditn]

so im thinking of installing a wider axle and im looking on information what will happen with handling.
wheels are 13x6 et 21.(ae86 original 13x5 et 1
i have to use those wheels since im making replica rallycar

front has 25mm spacer for brakes
rear would be +40mm wider on each side.

effect would be twitchy in corners
braking
steering efford
bearings
..?

how bad would it be,handling is now good dont want to lose it.
Is it big impact,only something for expert,...

this is info from forum
Wheelbase
2400 mm / 94.5 in

Track
front
1355 mm / 53.3 in

rear
1345 mm / 53 in

Length 4180 mm / 164.6 in
Width 1625 mm / 64 in
Height 1335 mm / 52.6 in