Some companys do not like modifications, and some don't mind, best to enquire with several companys, and tell them your plans, and if they would be ok with that.
If they say no, on to the next company.

My old wagon I had insured with RAC, and I told them i was putting on a new Turbo, FMIC, TBE, Fuel pump, lowered and ecu. They were fine with it, and it didnt cost me anymore, plus value of parts were covered!
I have been with RAC for a while though....