These truck motors came with 2 types of heads: straight-valve head, and a tilt-valve head. Tilt-valve head has a wider valve [rocker] cover. ALL SD's were straight-valve heads, as far as I know. EARLY Black Diamond motors also were straight valve motors, but later BD motors were tilt-valve motors. Obviously, the tilt-valve motors were considered an "improvement" over the straight-valve motors.
The straight-valve motor is still probably an improvement over the ag engines, but probably not for the reasons you might think. The truck motors were set up for a max of 3600-3800 rpm, so I wouldn't be surprised if you ran out of flow there on a stock set of ports. BUT the combustion chambers are supposedly smaller [and shaped differently] than the ag motors--not that I've CC'ed either head--and would raise your compression ratio.
On the OldIHC truck forums a couple of years back, some guy was mixing and matching parts between a straight-valve engine and a tilt-valve engine. Apparently he used pistons from a tilt-valve engine with a straight-valve block and heads...and wondered why the motor overheated and pinged. Turned out the static compression ratio ended up around 13:1, based upon the compression numbers he was getting. Not saying you'll get anything close to that...but I've heard that using the truck heads WILL raise compression over using the ag head.