I remember the thread, but could not find it again.

Do you have a pic of the locomotive you want to model?  If you post a picture of the locomotive you want to make yours look like some of us might have an idea.  It is generally not hard to enlarge an Athearn fuel tank and keep the motor mounts in place, but a picture of the prototype gives us an idea how much bigger you want to go and where it needs to be bigger.

I see the problem...(and some possible solutions...)

Dear Craig,

It took a bit of looking on HOSeeker.org, but I think I now see the issue.

There is a small horizontal tab on the frame which engages with a matching slot in the plastic shell,
in a vertical plastic wall which is meant to be the side-wall of the smaller square-r fuel-tank,
with a molded half-relief air-tank cast on top...
(see highlighted pic below, C/O and with respect to the team at HOSeeker.org!
You guys are an Invaluable Resource!)

Now, this also, IMHO, gives a clue to solve the "need a bigger fueltank" problem. In short, carve/file away the "half-an-airtank" molded bulge, leaving just the "vertical plastic wall" section, and the key "slot" which engages with the frame tab...

...then simply glue onto the plastic wall a 3D print (or equivalent) piece which simulates the larger-fueltank visible contour, taking care to glue the piece flush with the bottom edge of the slot,
OR
Designing/forming the "faux fueltank profile" piece with a suitable recess that aligns with the slot, such that it does not stop the frame-tab fully engaging with the slot...

...but now that I think about it, another alternative is to:
- snap/cut the entire "below the frame" faux fueltank/air-tank pieces off
- Trim away the existing cast-as-part-of-the-frame coupler mounting pads, and convert the loco to "body mount couplers" .This uses a standard Kadee #5 draftgear box to both mount the coupler, and retain the frame within the body shell, thus removing the reliance on the "fuel-tank tab/slot" shell<> frame mounting system.

- then glue/modify the "newer larger fueltank" (KV Models brass unit?) directly onto/over the metal frame "fuel tank" area...

Anywho, whichever way you elect to go, it looks very do-able...

Happy Modelling,
Aim to Improve,
Prof Klyzlr

Link to Criag's SW1200

Russ,

Here's the link to his thread.  https://forum.mrhmag.com/post/dynamic-brakes-for-emd-switchers-12219611 It'll show you the engine he's talking about.

Bernd

Doable

Slice the half-round that represents the air reservoir off the large tab, then build up the fuel tank over it.  Some thick Evergreen styrene sheet filed to shape should work.

Scott Chatfield 

I don't understand?

I'm a little lost here, I do body mount my couplers but if you cut off the frame lug off I don't see how you use that to connect the body to the frame?

I'm a little confused here, Craig

Coupler Mounting

Use the Kadee coupler box to hold the body to the frame. 

1. Build up a mounting pad using styrene to attach the coupler box to the body shell using a 2-56 machine screw. Note. The styrene mounting pad should only extend to the point on the inside of the body shell to where the horizontal surface ends. Thickness will be "about" flush with the top of the coupler opening on the body shell.

2. The original coupler mounting on the metal frame must be cut off. Cut the metal frame so that it now fits with a snug fit between the two new mounting pads. The original mounting tabs should be filed off smooth.

3. The rear end of the Kadee coupler box will extend back over the metal frame, thus with both couplers installed, the frame will be held firmly in place. 

Note, a fair amount of file work and test fitting will be nessessary to make this work. The advantages of spending the effort are the body shell can not be overstressed or scratched by spreading the bottom over the tabs on the frame, the couplers are now insulated electrically and a better detailed front/rear pilot can be constructed.

With the removal of just two screws, the body can easily be lifted off the frame. Glueing the coupler box together makes for much simpler assembly, but do not glue the coupler box to the mounting pad!