For those modifications you want to make i cant see there being a problem as you a) will still have front bumpers (aslong as you wernt thinkign of just placing a metal bar there) and b) in my 1990 nissan patrol i have only 2 seats and there is no problem with registration just that on your car i.d card it will say the car is for 2 people meaning if you put the back seats in again unless you go and get the card changed your car isnt insured for any more than 2 people as far as im aware

hope that helps