I've got a 351M that I bought for my truck. I put it in and run it some and it's smoking pretty good. Mainly after it warms up, so seems more like a ring issue. I ran a compression test yesterday on it
1 - 100
2 - 110
3 - 105
4 - 97
5 - 100
6 - 110
7 - 105
8 - 115

I put a few drops of oil in the 97 cylinder and it went to 120. The engine seemed to run pretty good, just lots of blue smoke. I don't really want to spend a ton on the engine, don't really care a lot about power just want a good reliable engine that doesn't smoke. What are your thoughts on what I've got?

 

I think that you have a tired 351M.

Being a 4x4, and seeing as you have installed it, you could do like a dealership did on a car under warranty and worked well enough ... pull the heads ... get them done while you pull the pistons ... hone the bores ... re-ring with new cast iron rings ... re-install with new rod bearings ... and put new main bearings in one by one A HV oil pump with ARP drive shaft wouldn't hurt, nor would likely a timing chain.

Or you can yank it back out.

 

Replace it with either a 400 or a 460.

 

Sounds like it's time for a rebuild. I'd pull it out, tear it down and see what you've got. until you look things over and get some measurements it's all speculation.

 

X2 on the 400 or 460 route. My 79 originally came to me with a 351M and it served me very well with some performance upgrades to get more oomph out of it. And I was happy with it. I got 180k with it until a leaky fuel pump ate the mains in short order so when it came time to rebuild it I opted to build a 400 that way I could reuse the Eddy intake and Cyclone headers that I already had. Really wanted to drop a 429 in it but marital sitch/finances dictated otherwise. Still wish I'd but the bullet and done it.
So I'll start this part by saying I did the rebuild in 1996... it was just a basic rebuild to get a good reliable truck engine. Nothing crazy. It was bored .30 over, crank .10 under..all new bearings, rings etc. The 351M had a Comp Cams 268 bumpstick that had a slight lope. More than a couple times I had dudes asking me if it was a 390. Hehe. Went the same route on the 400. Worked good. Ran good and was very reliable. I also retrofitted the valve train with a screw in stud kit so I could run roller tip rockers. The kit was made by Crane...don't know if they still offer it but it worked very well. My only regret there was not using the right rocker ratio for the 400. A local shop mentioned to me that BBC rockers would work just fine for the street and they did but I still wonder if I'd got a little more lift as the ratios were similar. (1:70 vs 1:73) So I'm thinking I left some on the table.
I think the biggest decision for you will be choice of camshaft. The days where you pick one off the shelf and install it are gone. Break in oil is crucial since there's not nearly as much zinc in it nowadays. Roller cams are a thing now and the break in isn't nearly as critical with today's oil choices. There's a lot written and argued about it. Chose carefully and go through this only once hopefully. Side note ...when I dropped the 460 in my SCS...I really wanted to put in a slightly bigger cam but didn't for fear of wiping a lobe at break in. I've never dealt with putting a roller cam in an older engine so I figured the stock cam would be fine for me. But I did put in a new timing chain.

 

Depends on what you want. Keep driving it as it is, and you'll never need to have an oil change again.
I was in the same boat when I found some broken/bent/missing push rods in my 351m.
I ended up swapping it w/ a freshly rebuilt 400 that I found on FB marketplace.
swapping for a 460 sounds good until you find out you need different motor mounts, flywheel, pulleys, exhaust, etc.
I went for the easier plug and play option.

 

I do have a locked up 400. Not sure that I want to go to the expense of new pistons and machining the crank.

 

You might be doing that to your 351 anyway. until you tear it down you don't know.

 

That's true. I've got the cab and and front end off right now, so I guess I might as well go ahead and pull it out and put it on the stand. See what I've got.

 

Absolutely. it's 10 times the work to do it in frame and most likely you're going to have to pull it anyway.

 

I would tear down your 400 too and see what it needs.
If they both need the same amount of work I would definitely go with the 400.

 

Since you're in for a rebuild either way, I'd rebuild the 400. With just a little bit of care you can make 300HP. Camshaft and timing chain, headers, 4bbl. carb. - easy.

 

I pulled the engine and have started disassembly. I've got pistons out, and the cylinders look very good as far as scratches and such. They do have a pretty good ridge at top though. I can catch fingernail on it. I don't have a bore gauge, but am going to try to measure with the ring end gap method. Engine hadn't been apart before. If I can catch fingernail on ridge, what's chances of getting by with just a hone? I'm thinking I'll probably have to invest in a bore

 

If you can catch the ridge I would plan on getting it bored if you want to do it right. but if you got them out without ridge reaming it's not terrible, you might be able to remove the ridge and hone it if you want an economy rebuild but until it's measured and checked for out of round you won't know.

What is the bore size? if it's been bored already the pistons will have it stamped on top.

What did the rod and main bearings look like and are they oversize?

 

It's all factory on bottom end. Rod bearings down to copper, but nothing crazy. Crank feels smooth. Haven't pulled main caps yet