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Almost all trucks of that era were under powered. If you live in Kansas keep the 225. If the truck will ever see a hill with a load use a 318 at least.
Yep, the hills kill my 1982 W150 4X4 slant six. No problem at all cruising 70 or 75 here at 5500 feet in New Mexico - but once you hit the hills it all starts to s l o w d o w n. 
One issue is the gap in ratios between 3rd and 4th gear as the truck has the granny 1st which I never use so essentially it''s a 3 speed. 45-50 MPH in 3rd is a menace here where average speed on the interstate is 80-85 MPH. Gas mileage is mixed 13-14, 16 highway. Engine has stock exhaust manifold, modified exhaust (exits under driver door). offy 4bbl manifold, Carter 400cfm 4bbl and GM HEI ignition. Ported, shaved (.070) oversize valve head to go on sometime hopefully soon. Torquer cam to go on a bit later.