4-color black and white images, GCR and Custom CMYK

Hi, I've been asked by a printer / client to adapt a 4-color black and white image using heavy GCR to prevent any color cast and ensure the K channel is dominant. I've been advised to do this by using Convert to Profile / Custom CMYK and adjusting the GCR black generation to Heavy. I've also read elsewhere that Custom CMYK is some sort of 90s aberration and it's better to pull out your eyes than use it. What's the best way of getting good results here? They're only asking me to do this for the black and white images in 4 colours. It's using FOGRA52 uncoated (side-note: converting to a custom profile doesn't include this as an option…).