I have KYB shocks on my 83D150. They have about 45,000 miles and 10 years on them. The KYB’s were pretty stiff early on, but softened up some with use and mileage.
Probably not a bad shock for what they cost.
I have Bilstein shocks on the 68 Barracuda. This is a different vehicle than the D150, but it is easy to tell that the Bilstein shocks has much better valveing for vehicle stability and for reducing the effect of road undulations.
Basically the Bilsteins are a much better shock. But they are also a much more expensive shock.

Contact PST directly, they are a Bilstein shock distributor and would know what is available.
PST is a FABO sponsor and they give 10% discounts to FABO members, you need to mention the discount when you place the order.
I have a front sway bar on the D150 that I got off an 85 D250 Camper Special that I found at a pull a part. I got a rear sway bar off the same vehicle but have never installed it.

I haven't had good luck with either Monroe or kyb over the years.
The best shocks I have found have been Gabriel. I run their "ultras".

I'm a fan of the KYB Gas-a-Just monotube shocks. That's their medium duty model. I've had them on several trucks over the years, after the oem or Monroe's failed. I currently have them on the rear of my '82 D150. I had Monroe Gas magnums on the rear, but they were sacked in two years/10k miles. They also rode terribly. I did put new leaf springs on last year as one of the leaves was cracked. Replacements were the 4+1 lt. duty 1/2-inch eyebolt, "318" springs, replacing the "slant" 3+1's. I've had Monroe oem spectrums on the front for 4 years and they have been "OK". I do want to replace them with the KYB Gas-a-Justs as well, especially having a sway bar.

there is no such thing as "slant 6" and "318" leaf springs. in any range of trucks (100,150,200,250,300,350 etc) there were several "GVW" packages. Most were not dependent on engine size. Front coils? Fron the factory I think there were. In the Moog catalog they were grouped differently, I think /6 and smallblocks were clumped together.
In years they had both a "100" and a "150" the "100" was the lightest package GVW and the "150" was known as a "heavy half ton" or some called it a "5/8 ton".
At one point it was a way around adding emmissions "equipment"--or not.

for instance my current truck (emblems gone I think it's really a "D100") has a 4800 GVW. I have seen some "D100s" and "D150s" with as high as 6050 GVW. The biggest difference? An extra leaf on each side, in the rear springs.
I had an 83 D250. There were 3 levels of "D250" in 1983. One was ~6900, mine was 7500 and the heaviest was ~8600 GVW/ had nothing to do with what engines were/weren't available that year. The lightest variant had a 9-1/4 rear end with 8 lug wheels, while the 2 heavier ones had Dana 60 rear ends.