It should'nt be too hard or expensive to dial out most of your understeer Ty,but not before the weekend.
Firstly you need to find a truck spring business,where they can cold bend them.I had a pair done this week for $88.00 inclusive.
There must be one of these places around the capital with all the heavy transport in the area,may take a few calls.We are talking leaf springs ar'nt we?Once you find someone get in contact before starting anything,
Tyre selection won't fix a problem,but will mask it for a short period until the tyre overheats and "lets go"but i'm sure you know this already.
Good luck in the mean time for the w/end