Do a search for replacement springs, you'll find ONE part number for stock replacement springs for D100, D150, D250's w/o 4000lb axle, and NO mention of engine size. There are heavy duty and severe duty springs offered, but again, ONE part number for each of those, for ALL D series 100/150/250 trucks.
I've driven/hot rodded/researched D series Dodge trucks for 20 years. Here's a pic of my '84 D100, was a factory slant 6 truck. 440 motor, with one coil cut off the the stock 6 banger springs. About a 1.5" drop. Will a big block squat the nose more than a slant? Of course, but Ma Mopar was too cheap to make a specific rate spring for every engine combination. Research further, and you'll discover that 1/2 and 3/4 ton trucks use the same upper/lower control arms, ball joints and tie rod ends.
And for what it's worth, add a factory front sway bar and the understeer problem is greatly reduced. These trucks plow and wallow like a pig with no sway bar regardless of engine, though admittedly, the more nose heavy it is the worse it gets, for obvious reasons. If you have manual steering, ditch it. I've gotten this truck crossed up and it's nearly impossible to crank the slow ratio manual steering quick enough to get it gathered up again.