How are you going to mount your seatbelt? Pretty sure that seat is meant to be fit with a harness, and without a cage or a properly designed rear bar, you wont have a solid mounting point for the harness.
What i mean is, if you have a properly designed mounting point for the harness, the weight of this mounting point will negate any saving that the seat will save.
Cheap, Safe, Functional. Pick two..
Edit: Evo Recaro from 4-9 will fit if you change the rails, width-wise..