does anyone sell a factory style fuel tank with a sump built in already? I cant seem to find anything for my duster

Probably not
I had a local custom car builder shop build and install one in a new Spectre brand tank for my 1971 pump gas street Duster.
Look around to find a add on sump you like and see if a local shop will install it for you on your existing tank or if they want you to buy a new gas tank first for their own safety
The shop that did mine used a round top off of a small (5 to 6 inch diameter)automotive air conditioner reservoir I think, they welded in a 1/2 NPT bung and I ended up using a 1/2 NPT to #10 AN steel nipple in the bung to feed my fuel pump mounted behind the sump. The sump stuck up into the tank around 4 or 5 inches and had a number of 1/2 inch holes drilled in it to allow fuel to get into the bottom of the sump, it work very well

what should be done to an old tank to weld a sump in?
just clean it out with some degreaser or something? maybe fill it with co2 and seal it up before welding??

Too bad Rockauto doesn't list tank with "in tank pump" already built into tank and sending unit . . . they do for B body, so for those looking to upgrade to FI on a B body, change out the complete tank and no need to mount external fuel pump . . .

I believe they fill it with water.

Why the sump? That's ok for a dedicated drag car if the sump is properly placed and plumbed correctly, but for spirited street or road course driving; it's less than ideal.

If you are trying to keep cost down, add a return line to your sending unit, plumb a small lift/pusher pump just outside the tank to a small surge tank, and plumb it on up to your motor.

A simple surge tank system will deliver air free fuel down to the last gallon and out perform a sump set-up every time, especially if you run a Holly Hydra Mat in the tank.

I've been looking for the same thing for my Demon and could not come up with anything.
Closest I could find is to buy the sump itself and weld it in.
https://www.summitracing.com/parts/cee-4...ASABEgId4fD_BwE

Luckily I found a guy at our local speed shop that will make his own and weld it in to my (new) tank for $150.

I have been running that Summit sump on a factory tank for several years & it works great. The only complaint is very limited ground clearance if you have to put the car on a flatbed truck, facing either direction.

I used a hardware store hole saw to cut holes into the bottom of the tank and a local radiator repair shop welded the sump on.