1990 Ford F-250 3/4 ton 351 Mileage 150, 00

Last week I shut my truck off and it would not start again. I had it towed home and replaced both Fuel Filters, plus the rear gas tank. After replacing the fuel filters the truck will still not fire on the rear tank, it will fire on the front tank but it takes quote sometime when cranking it, and you can smell the gas. I was told to check the fuel injectors, how would I do this, and do you think this is what the problem would be? Thank you for any help!