Not sure what you already found, but seems to be quite a lot on the net.
I have seen a few heathkit adverts in eti and practical electronics, but they were mostly adverts for the heathkit catalog. I had heard of the H8 but not sure if this was in a review or advert from the time or something I have seen more recently.
http://www.decodesystems.com/heathkit-h8-ad-1.gif
Schematic for ram and crt here:
https://www.vintage-radio.info/heathkit/
http://www.oldcomputers.net/heathkit-h8.html
https://wiki.theretrowagon.com/wiki/Heathkit_H8
https://museum.syssrc.com/artifact/118/