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1981 C20 Pickup LS Swap Guide

Updated: Mar 27

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*This article is being constantly updated, so check back often for progress

In Early 2019 I became the owner of a fairly clean 1981 C20 Pickup.

The truck is a long bed, original 454/th400 truck.

Unfortunately for me the original engine wasn't included, in fact it had what turned out to be a junk 305 with pistons rattling around inside the bores.

How I got this truck was, i was doing a side job in the summer as i do every now and then, the task was pretty straight forward, source and install a Small block 350 into this truck.

Well, due to my arrogance I ended up accidently getting said 305 instead of 350. Me being a trusting guy picked up an engine from a fella late at night, taking his word that it was a 350. Im no expert on Small Blocks, in fact I don't claim to be an expert on anything, so I installed the engine, got it running and boy did it smoke... Alot.

So doing some initial troubleshooting, i noticed the valve seals could probably use some life, so i replaced all 8 of them, (This is one of those jobs where having the right tools helps so much.)

This resulted in lots of frustration as small block rocker studs are very fragile compared to the LS ones im used to. i bent a few using the valve compressors and had to spend hours bending them every so gently back into place.

Heres the video i made on that subject:

Long story short after a few bent pushrods and lots of cuss words, i got the engine dialed in really nice.

But it still smoked... ALOT

So me and the owner decided lets cut our losses, I gave him back the Labor i charged + 2500 dollars in exchange for the truck.

So now I had this truck which I didn't really want, so I figured I would drop another couple hundred on an actual 350, drop it in and sell it, maybe earning a few bucks on the end.

It was to my surprise when I could not find a single decent 350 for less than $2000 in my area, everyone under $2000 had a ton of miles and every one under $1000 basically needed full machine work.

So the truck sat.

Regretting this truck sitting in the yard, I started looking into the Iconic "Squarebody's". I thought screw it, i might as well cut the long bed down to a short bed, as i always thought only short beds were worth anything. Then i decided, lets LS swap it. Its what I know best

But instead of "Ruining" the truck by chopping the bed I decided to roll with the Long Bed and just get the swap done as simply and inexpensively as I can.

In episode 2: We start prepping the truck for the swap

Now Originally, I had intended to use a 4.8 engine I had laying around, and I explain that a bit in this video:

But I ended up breaking a few more pistons on a press that was far too small for this job,

I proceeded to build the engine, as you can see in this video:

But after putting everything back together, the engine wouldn't spin freely, I believe what happened was when I was putting everything together, and after buying a new(used) rod, the clearances in one of the rod caps was not correct, either the caps got switched, or some other issue occurred. I'm pretty sure a new set of rods would solve this issue, but then I saw something I couldn't pass up.

I saw a 1999 5.3 Chevy Silverado on marketplace in my city for only $2000, i bought it an hour later for $1500.

This was now our donor for the truck.

Since we were in the Silverado for only $1500 I was sure I could part out the rest of the truck and make my money back, if not make some extra money.

Also this gave me an opportunity to "redeem" myself, as a lot of people were upset that i was using an engine I already had and counting it into the "affordability" of the build. Now I doubled down, and showed the doubters it could be done for "free"

(Yes I know its not technically free, but if you buy a truck for 1500 and sell everything but the drive train and make $1500, the drivetrain didn't cost you anything.. In other words, Free)

Here's some videos showing the disassembly process:

So after that, i parted out the entire truck and made roughly $2500 and scrapped the rest.

Now we had about $1000 extra for parts for the truck, not bad at all.

So lets get this puppy installed:

Now in the video above, I used the cheapest LS to SBC mounts available, these worked, but weren't perfect, I wanted a better fit for the engine. so these are what I ended up using:

And then for one of the more "complex" aspects of the swap, the fuel system.

Surprisingly through some research, the fuel system actually isn't terrible to do on these square bodies, thanks to GM switching over to EFI in 1987 a drop in gas tank is readily available for around $100. I also got that years pump assembly and a new Walbro pump, threw them in the tank and then opted for AN6 line to go from the tank up to the engine.

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