Been there done that
i had a vanguard v450 as ADS said 40 hp mariner and if you want a longer range you have to fit a 25l tank next to the old one i did that as well
and i made a plane the rib was to say sky happy

the only way out
is to fit a jockey consul and keep the rear bench in place which will balance the craft but the gp hull is still not the best its built for the med not the british weather if you add a new tank with the existing seating the boat will slip down any waves and the prop will not grip bringing you to a stop which can be quite bad
i ended up with about four stone of lead in the anchor locker in the vanguard move the battery in to the the locker by your feed it should never be in with the fuel tank
if you move the seat and steering forward watch out as the gp hulls are solas type and are sealed with no drain any holes in the deck not sealed will
allow water in to the hull with no rear drain it will fill up and wreck the boat
in the end i sold it and went for the Dr450 with a bench and jockey
and a 50 hp which was fine and would reach 34 knots
as you can see from the pictures in below taken flat out check the bow height
Daniel