I, too, am addicted to GPs.
Lionel did make several types of rolling stock which are in the neighborhood of 1:48. Most of them were introduced in the mid-50s, and therefore were not produced in great quantities in the postwar era, but that tooling was well utilized by MPC and LTI. There are two sizes of refrigerator car, and even the smaller one is noticeably larger than a 6464 boxcar. There is the quad hopper, the large stock car, the chemical tanker and the 3 dome tanker (and the early postwar single dome tank car lettered for Sunoco). Flatcars and gondolas would be about 40' long in 1:48. Even the tiny 2-bay hoppers are not that much undersized compared to the tiny real hoppers of the time, at least in terms of length, though they are definitely too narrow.
Even the 6464 boxcar could pass as a scale-ish 36-foot car: MoPac, for example, resheathed a large number of old 36' boxcars in steel and painted them for LCL service. These cars were only 9 feet wide at the eaves, and 12'10" from the rail to the top of the roof walkway. They served until 1960. The MPC 6464 clone boxcars (all I have) are only about 3 scale inches shy of these numbers. I don't have stats for length, but the Lionel car scales out to 39' (including the roof walk), which I imagine is about right for the 36'6" interior length of the real car. So, in overall dimensions if not in detail, the 6464-150 MoPac Eagle car is actually close to scale.
If you want a scale 40' boxcar, you could use Menards'. Or use older Lionel Standard O, though these sit annoyingly high - I've often contemplated lowering them by replacing the "detailed underframe" with a recessed block of wood, but I'm too much of a "collector" at heart.
Cabooses will be a little tougher, since you can't use the ubiquitous SP type. The porthole and bay window cabeese are close to 1:48 (though the bay window is based on an Erie prototype, which is admittedly narrower than many other real bay window cabooses). You would definitely have to go with modern Lionel (or even non-Lionel) to get a 'typical outline' caboose that really looks right with a GP.
But, unless you need to have all your rolling stock be Postwar, there are plenty of options which will look good with Lionel diesels, other than the FAs. Of course, all your steam engines will be undersized then. Still, real steamers came in a lot of sizes, after all...
