to me, hpi makes some of the best looking touring bodies (not looking at aero dynams and such). but you have discovered the painful truth, these things dont mount very happily on 710s. here is my simple formula for mounting them...
1. measure your wheelbase (about 259mm on the 710)
2. measure the distance between the center of the wheelwells (256mm on the body).
3. do the math - there's a 3mm difference, so you need to mark 1.5mm from the center of the front wheelwell towards the back of the car and mark 1.5mm from the center fo the rear wheelwell towards the front of the car. these marks are what i actually use as my wheelwell centers.
4. from there i set my circle cutter to wheel_radius+2mm and position the center point of the circle cutter so that it is on my mark. BUT!!!! if the blade is not touching the upper line of the wheelwell, i move the circle cutter up so that blade DOES touch the upper line of the wheel well - this means that the center point of the circle cutter may be slightly straight above the mark i made on the wheel wells.
5. then i cut it out.
using this "method" with hpi bodies has produced good results for me. it may not fit PERFECTLY, but after all is painted and done, it still looks very good to me.
the BMW fits better than the Vauxhall (shorter wheelbase on the Vauxhall). so far i have had several Vauxhalls and only one BMW.