Cool project!
Having built several log-booked cars for Rally-America, I'd advise taking the time to carefully learn the rules governing construction -specifically safety items. Example: You will need a pair of 6-point HANS style FIA approved harness'.
There are also special logistical concerns native to rally such as stage rally has a lot of miles (both transit and stage) between fueling. I'd suggest a 14 gallon minimum cell, but honestly, if you are going for max cost savings, re-use the stock 16 gallon tank.
Anyway, if you're not already there, start a thread in the car construction forum over at
specialstage.com The people there can give you great advice to help you construct a legal RA car.
Good luck!