I came across that page a few years ago, but could find nothing further on the vehicle or the tie between the two companies. The best I can tell, Bellco is a coachbuilder that leans toward building sports racers and open wheel race cars (or most of their work in the 1970's was in this area). Between the aluminum rebodied Buick and the Mitsubishi rally cars, everything appeared to be sports and open wheel. Now they do plastics.
In the race results, there is a Bellco 72A running against the Bellett R6 cars. This appears to be the Suzuki powered car mentioned in that time line:
This had led me to the conclusion that the mention on the time line had more to do with the development of the R6 cars and less to do with the Bellett GT road car.
But that conflicts with the later date of the R6 cars showing up (1970), the stories that the R6 was built in house by factory employees, and neatly fits into the time line of the Bellett GT introduction. The only problem is that even in the Neko Publishing book, there is no mention of Bellco working with Isuzu anywhere.