OK not what you're asking about but I'll throw this out there for you to consider.
About 25 years ago I bought a set of blackjack headers for a 360 in my '85 D350.
There was an article in Mopar Action about a towing build-up that used them.
They didn't specifically list them for a D350, but I figured I'd give them a try.
If I remember correctly they interfered in two places. the main one was the top of the frame on the drivers side.
The welded crossmember also hit and some tubes had to be hammered.
I believe the frame on the 350 has a deeper C channel shape, which makes the top of the frame wider than a 150.
I eventually got them clearanced but really hated beating on my new, shiny, coated headers!
Hope you don't have this problem!

Just looked at the listing, it DOES NOT say it'll fit a 300-350 series truck, buyer beware!