i install FPR to my old kancil.. either way is ok.. the stock regulator only regulates on certain level of pressure. install adjustable FPR will increase the pressure more than stock. i did both of the way last time with my kancil but no big different.
if u install FPR with stock regulator, u will have safety so when your FPR kong u still have stock regulator (ori FPR rarely kong). If u cut & weld the fuel rail, no safety but better reading of FPR gauge. Or maybe u can go get WORKS fuel rail instead bro?