I bought a 85 D150 earlier this year, with a slant 6. Id seen this truck sit... and sit... and SIT some more. I assumed it was probably a 318 truck til I went and looked after a "for sale" sign popped up in the window finally. the clean body is mostly what drew me to it especially for a northern Indiana truck.
I figure by the time I buy V8 truck mounts, a V8 trans, a V8 radiator, and rebuild one of my V8s I have here, I can build up the slant 6 to run pretty damn good.... Only a 1/2 ton, even if I put a 1 ton suspension in it, legally can still only haul or tow what this particular truck was rated at from factory. And it's the lightest GVW, even among the 1/2 tons of that year. I have had slant 6s that surprised me, for how well they could get around/ and yeah, I have had a couple of slugs, among those I have had over the years, as well. ....
I have a 318 in a 78 Sport Fury that as of now is a slug as well.....(not for long.... I have some "goody" parts on hand to wake that one up, too but that 318 in that car is how it came from the factory) the slant in my 80 Volare and my Moms old 79 LeBaron did surprisingly well. More guts than would be expected from either, totally stock.
Just a good tuneup, valve adjustment and super 6 swap, made a HUGE noticeable difference in my son's D150. though it was neglected before he got it, the tune up plus borrowing my 1 bbl from my truck even helped..... the super 6 helped more. I want to pull the head and do some work on it.... mostly a shave. That will help that truck, even more.
don't discount the 6 so fast. There are guys running 12-13 sec 1/4 mile with them..... though these are mostly no longer streetable. gearing also makes a difference.
I went looking for a Super 6 manifold stack, one for me and 1 for my kid's truck..... I got my 1st one then shortly later found the 2nd, with the rest of a basketcase 225 "along for the ride" came with.... with the money I am not having to spend on different trans, different radiator and such mentioned in beginning of my post, that's more that I can put into this 2nd slant that I have on hand to "see what I can make it do"....and drop it in there. If it still "aint quite enough" I can transplant into my Volare that I was originally wanting to do the V8 thing to. Ive done the "yank the 6 and drop a V8 in" on Dodge trucks before.... before I knew much of what could be done to improve on the 6.
In the mid 80s, even the 360 wasn't rated much over 150-160HP from the factory..... and they moved such equipped trucks along pretty decently.... 305/350 Chevys and 302 Ford of the same era weren't barn burners either.... at least as they came stock.
Without getting too crazy, I can get that much out of this slant 6 and even more.... with gobs of torque from that long stroke of the engine.
Then you have the old "everyone puts a V8 in" thing..... I'm trying to do different with this one.
I have a 440 sitting here that I "could" drop in, but Id then have to find some of the above mentioned parts because comparable ones for the 6 won't work on the V8. and big block swap parts are becoming even harder to find than smallblock parts (and more costly) It's been longer since they put a BB in a truck than a SB engine......
You had said you haven't done anything but drive it home 50 miles..... have you pulled the plugs for a look? Adjusted valves? Adjusted carb? Cleaned/rebuilt (if needed) carb? Checked timing? Done compression test? Checked brakes for dragging? Id bet on alot of "no's" to these questions. between here and the slant 6 forum (LOTS of reading over there-- and here) dig around and see what others have done with the slant 6 before you bail..... it runs (somewhat) as is so that's a good part of the battle.