If this is the original engine, then there should be a sticker with emission control/belt routing etc info located under the hood, near the latch. The engine displacement (5.0 litre for a 305, 5.7 litre for a 350) should appear on this sticker. I'm not sure how to go about deciphering it, but there should also be an option code sticker (in the glovebox? - I know that's where it was back in the '70's!) that may tell you the engine it came with.
To determine which engine it is once & for all, find the block casting number. Look on the block, on the top of the flywheel housing and just behind the driver side (1-3-5-7) cylinder head. There is a number cast into the block which will tell you which engine you have. A bit awkward to read with the engine in the truck, but you can do it - a good flashlight helps! It may also be necessary to clean the number off with a wire brush.
Unfortunately, none of the books I have list the numbers past 1986 (since everything I have is old/ancient (depending on what you consider old!
)) but I'm certain if you get the number off your block & post it here, someone out there will be able to tell you exactly what you have.