Someone on HN reminded me of how they went about making the XT/370 and AT/370: they wanted a 32-bit "mainframe on a chip", so they got the Motorola guy who wrote the 68K microcode to create a customized pair of 68K CPUs which implemented the System/370 instruction set. Then they similarly modified an Intel 8087 for the FPU, plopped all 3 on a card, and put that into a standard XT or AT.

(https://www.cpushack.com/2013/03/22/cpu-of-the-day-ibm-micro...)