The best thing to do is check and see if your truck was produced with the 250 six in it in the first place. If so, then all the necessary parts to do the swap are easily available. I am curious why you are wanting to go to a dimunitive 6 pack in place of the 350.
If its for fuel economy reasons, there are a lot of things you can do to improve the fuel mileage of the 350. I fear that going to the small 6 will prove to be disappointing in that it will require the engine to work a lot harder, and that equates to fuel consumption. not to mention that the 250 puts out no where near the torque your 350 does, to move that heavy load.
I'm looking for a temp engine to replace the 350 for the winter. Actually I'll take a 305 if I can get one. I'm just trying to see what I have for options since price is playing a megga role in this swap.
I've read that the strait 6 had put out 165 horse and 265 torque. The 350 put out 175 horse and 275 torque that year (1979).
I wasnt aware the 350 was that detuned. Doesnt sound like theres much difference in the output of the two. The figures for the six sound more like a 292 six vs a 250. You may well be about as well off to just find another small block Chevy of any size and drop in, since you already have everything there to hook it up, without any mods. To be sure, installing a 6 will require at least, changing motor mounts, and possibly having to move the transmission back, requiring shortening the drive shaft.
The 305 and 307 arent popular engines so if economics is a factor, you might want to go with one of them, due to their probably being a lot cheaper to purchase.
It's not as bad a swap as some here are making out at least for the 250 and 235. Power wise they're right though - you're better off finding another small block; and the power numbers you list do look like 292 numbers not 250 numbers. It's not a 100% drop in swap bu,
If you found a 250 or 235 then all you would really need to worry about would be radiator clearance, and carb linkages engine harness as it is, and the like. The engine mounts for the small and medium straght sixes (chevy) line up exceptionaly well with small block mounts. The 292 has one side's mount closer to the front of the block. Your trans would bolt up but the straight 6 block is longer and you may need to move your mounts for clearance issues (rad or drive shaft)
If you found a 250 super duper cheap then it might work as a stop gap but after you factor in your time and a new intake and or carb and the associated bits and pieces... I just can't imagine you couldn't find a decent running small block for less total cost.
The operative word is SHOULD work the other way around. The V-8 is two cylinders shorter than the I-6 in length. There is little you would have to move in order to put a V-8 into where your I-6 was however, from the factory, the radiator for the 6 is located further forward etc. I still advocate the idea of going with any other sbc, simply for the ease of swap. Its all bolt in. Why go to all the trouble for a lesser engine?
as far as the 6 bolting to the tranny no problem watch out for the starter and fly wheel if u want it to turn over fast use a 168 tooth fly wheel with the v8 starter the inline 6 uses 153 tooth as for the mounts they may have to be moved closer together the only way i know is i did a inline six to v8 swap on my wifes truck after the 6 blew up the six will make better low end tourq than the v8 ever did
the six banger is a lot more stronger on the bottom end 7 mains versus 5 thats a no brainer and i have turned them harder than 7000 rpm in stock form the sixes come in 194 250 and 292 some in the early 80's came with a 2bbl but have an itegrated intake cast on the head and are not seprable so u do have some options they also like gearings from 3.42 on up
The backside of each engine is the same as far as bellhousing & crank bolt patterns. Your original starter & flywheel will work fine. I don't know about the motor mounts, but someone at the 67-72 site should.
The horsepower rating of a late-'70s 250cid I-6 was more like 110 or 115 hp. I had a '77 Nova with a 250 & a '78 with a 305, and they both got about the same gas mileage, but the 305 had more pep (neither had a lot, & the 6 would scare me in a truck). I'd get a cheap used 305, since it will bolt in without messing with accessory brackets, mounts, etc.