Dell Precision T7400 and T7500 Case

They absolutely are NOT identical or interchangeable.  The cooling and Mounting and CPU's etc totally Different.  The 7400 has 2 sockets.  The 7500 uses a Riser for 2nd CPU and Ram.  Mounting and cooling etc as well as front panel is completely different.

Thanks for the reply.  Just to clarify something: when looking purely at the bare chassis of each machine, is each chassis a different physical size with completely different fixing holes? I got the impression when looking at the motherboards, that they are the same size, with the same fixing holes but the circuits are obviously different but with certain things that always line up e.g. the expansion slots line up with the cutout in the chassis. Hence, I thought you could move the entire guts of the T7500 machine e.g. motherboard, fans, CPUs, plastic shroud, PSU, drives - everything - into the T7400 main case plus move the font panel over from the T7500 too.

You seem to be ignoring the FAT FAN Sticking out of the back on one case and having no fan whatsoever on the other case.   You are also ignoring the space in the front panel area and other things.

You buy it you try it its not gonna work,