Thank you so much for the information, Ben!!! You rock! I soon as I clicked on that Japan Wiki link and saw the VHS cover I knew it was the right film!
Despite what both sites report, the stone giant is referred as (or directly called) "Dai Kyojin" multiple times throughout the film by several different characters.
The movie's visual style hearkens to the other tribute films of that early 90's period, including the Hakaider and Kamen Rider (Zo & J) movies. I myself actually wondered, seeing it again as a 30 years older and more toke-knowledgeable fan, if Amemiya wasn't the director or somehow otherwise involved due to the similar look & feel of the film has; to Hakaider and KR 'J' in particular. It's interesting that you reported that it's supposed to be a 2-parter! I would love to see the other part. Having watched the film so many times it makes me wonder though if that info is correct, and I say that only because the film has a definite beginning, middle and conclusion. (But only seeing it 'raw', with no subtitles to more accurately inform me, I cannot say for certain.) The final shot has one of the film's secondary characters experiencing a Twilight Zone "did this really happen or was it all just a dream?" moment that I found cool and rather eerie when first watching it all those years ago. It also has a fantastic soundtrack, especially the main & end themes.
And it certainly does make me wonder how many other films we've missed over the years that never got a theatrical release, and thus are only known to Japanese fans.