if I'm not mistaken, a 1990 is kind of a hybrid year...roller cam, but LA heads, right? IF so, then yea, a 318 Magnum would give you a gain. the Magnum heads flow 200 cfm as cast, and can support up to 400 hp in stock form. I had a 318 in my 98 Dakota when I got it. had a 2bbl M1 intake, 220/230 duration cam with .507 lift on a 114 LSA, cheap headers, and a computer re-program via SCT (which only works on OBD-II computers)

my 4,000 lb truck was running 14.6 at 95-97 mph in the 1/4 mile with that motor...and I was traction limited, 2.2 - 2.3 60' times. (with 4.56 gears)

I dropped in a 5.9 with everything being exactly the same, except the cam. went from the 220/230, .507, 114 LSA to a 222/228, .544, 107 LSA cam, and went back to 3.92 gears. truck went 14.3@98-99 mph, with the same 2.2-2.3 60' times.