I have a older sub. Late ninties with the vortec 5.7. The forty something gal. fuel tank is going to need replaced at some point and will need a new fuel pump.
I do not need to huge tank in it and am thinking about putting a smaller tank in it. 25gal? Also would it need a different pump? Can I put an external fuel pump on it? Is this opening a big O can of worms?

Thanks guys.

It's going to open a can of worms. Why do you feel you will need to replace the fuel tank? It is a very rare event that a fuel tank needs to be replaced. It will be much easier to stay factory. Also most external fuel pumps still need an in tank pump as they do don't do a good job of sucking the fuel out of the tank, then you'd also need a pickup for it in the tank.

going rate in my sector for that era GM fuel pump installed is bumping $1000; don't see advantage to smaller tank even if OEM is damaged. You have a truck that sucks a lot of juice regardless of the use; smaller tank isn't going to change that except to make 'filling up' less pleasant/more frequent.

In shocking comparison, last spring I replaced my inherited 73 camper special 454 fuel pump, a model that predated the in-tank design. It cost $73 for the pump & minimal labor. Still has both saddle tanks, as 11 mpg is thirsty.

If it was a 60's truck I would say go for it. Sounds like a can of worms you would be opeining up. You have to consider mounting the tank with new straps and alignment of the fuel filler neck to the tank and would the pump be a plug and play or are there extra wires to deal with. Just a thought to consider.

Not worth the trouble, you might have to deal with the gas gauge not reading correctly also.