I have the opposite to you samwize. i have understeer on power. i lengthened the rear wheelbase to help it not do that and now i have the same problem as you. I'm going to try 1degree of anti squat and that should help it or even 6dgre Caster blocks. And i run Modified.
I run on asphalt and a its a medium grip track.
My setup is/ try this -
Front roll center have the 2dotted thing at the front and have the 3 dotted thing at the back. = 1degree of anti dive.
Fear roll center's have the 3 dotted things all round.
Green spring at the back and silver at the front.
And roll bars.
And no ackerman i think.
Everything else factory settings.