Well the weather finally broke in upstate NY and that means it's time to work on my '84 K10 Suburban (350 4bbl, SM 465, NP208). My current project is the fuel tank and sending unit as the sending unit is pretty rotten and started leaking. The current tank is marginal at best so I hit the junkyard this weekend and found a suitable donor tank out of an '86 Suburban. I got it out and realized that it's quite a lot bigger than my current tank. I did some measurements and it's the 40 gallon tank: my '84 came with the 25 gallon tank. From what I can see it looks like a direct bolt in replacement, needing only the 40 gallon tank straps and a 40 gallon specific sending unit. Did GM make any frame changes to accomodate the larger gas tanks? Has anyone tried this replacement/upgrade?

Same frame, different tank. If you get all the associated pieces with the 40gal tank it should bolt in place of the 25.

it should just be deeper but have the same foot print. i have a 83 diesel with the 40 and just put a 82 into my 67 other than height , they are the same

Great, thanks for the reply. I thought it would be a bolt in replacement but I wanted to make sure. I was wrong about a gauge cluster upgrade I tried on this truck as that fit but wouldn't work. I got the filler neck, tank, and hose from the neck to tank, new straps and a sending unit are on order, and I have some lines to fab up but I should be set.

I found another FYI on this project that would be of interest. To go from the 25 gal to the 40 gal I'll have to chisel out the rivets that hold the rear tank support in and move it back a few inches. There is another set of mounting holes further back on the frame that I'm guessing are for the 40 gal tank. The support bars are too close together right now to fit the 40 gal tank in. Unfortunately the trailer hitch has to come off and stay off to accomodate the rear bar.

this is why I like having the burb I do with a 40 already in it plus tow receiver for heavy loads :D
good luck!

An update on this post for anyone who may be interested. I got the swap and all the welding done over the course of the past 2 weekends. I ended up cutting the rear tank support out as the rivets wouldn't budge, even with the bottoms ground off. I took a 2" X 2" piece of angle iron, notched it to clear the tank straps, and welded it to the old rear tank support. Also none of the holes in the frame lined up with my new rear support location, so I ended up drilling my own.

The only oddity is the fact that the corner of the tank flange (top half meets the bottom half) is up against the edge of the C channel on the frame. It doesn't seem to bother it at all so I didn't worry about it. I ended up reinforcing all the body supports from the rear doors back and sunk a 3' X 3' piece of 18 gauge into the inside of the pass compartment to fix all the rot holes in there. The doors actually shut well now :)

congrats on getting the job done. i am still working on the front of my 87 burb, so i havnt gotten back to the rear, i do know that the tank is a 40 gal so i dont think i'll have to change it.

I did see you went to the junkyard to pick up some parts, i havnt gone to the yard over here yet but i heard they are expensive. How much did the parts run if you dont mind me askin?

Thanks. It was quite a project. The grand total on this project was $347 but that includes everything (MIG wire, all the consumables, steel, etc). The tank itself was actually dirt cheap. They charged me $5 for it because they punched a hole in the bottom I had to weld up. The filler neck was also $5.

The most expensive parts were new as the junkyard ones were marginal or unusable. The sending unit was around $100 and the tank straps were $20. If you live in area without road salt you can probably reuse the sending unit that comes with the tank. I also pumped about $100 of fresh steel and angle iron into my Suburban because all the body mounts in the back were shot.