Just purchased truck. Fuel pump was replaced before purchase. Truck will start, sometimes idle for 5 secs and shut off, or shut off when gas is pressed. Can sometimes hit the gas and get it to Rev up for about 5 secs and then it dies? It has 5 gallons of fresh gas and a can of Seafoam.

 

Reads like its time to rebuild the carburetor

If you take off the air cleaner will engine stay running if you spray gas or starting fluid(ether) into the carb directly?

If you can keep it running then issue is fuel flow from the Jets in the carb, assuming fuel pump is working
Or float bowl valve, but in either case carb needs to come off and be rebuilt

Not hard to do
2.8l V6 should have a Motorcraft 2150 two-barrel carb
Rebuild kits are available
Have a table with room
Take your time and take lots of pictures as you disassemble it

Google: 2150 rebuild

There are videos to watch

 

should be easy enough to check that the fuel pump is actually working. Remove the gas line going into the filter in front on carb, turn the engine over a few time to see if gas is flowing out. If not, could be a pump or restriction in the line to pump.
If gas is flowing out, i would think the next step is to remove the 6 screws holding the air horn on to see if there's a lot of grap in the float blow area. Is there plenty of gas in the float blow area?