What will be needed to change if i had to put the new Nissan 1400 front?

at a guess, you will have to change the old front.

but really, just the headlights and grille id reckon.

There are differences in the RSP where the headlights fit in.

There is a whole industry in Japan supplying components & even whole kits to convert late model 1200 utes back to the earlier round light fronts, but not one adapter is available to go the other way.

i am in the middle of doing this at the moment had a friend with square headlight front wanted round so we swapped made template of each frontout of cardboard had to modify headlight bucket hole as the square headlight goes more towards the center we got some 1.8 mm alloy plate and cut to shape and drilled holes so they would bolt up where side guard bolts go on and drilled and filed the wholes for the headlight adjuster aswell worked out good plus the grill mounts are differenton top raidiator support and they have lugs that sit in holes in top of front apron