89 dummy lights!!!

89 sierra SLE stepside...
just bought it, few problems already!!
when driving at highway speeds, as soon as i let off the gas for a second, the SES light comes on...
also noticed that the tank leaks when 40L or more are in it
just wondering how i figure out how many Litres/gallon tank i am goiin to need to replace this one....
not exactly sure wat size motor but i know its a v8...350/305? how do i figure this out too lol sorry guys

For starters for you, take the VIN into your GM dealer and get them to break it down for you.That will give you the specs you need on the truck itself.As for the ses, it will have stored codes,get it scanned.