The A-900 was an option for a few years...63 to 66?...and was intended for light delivery. The box was only about 4.5 feet long, so it's about right for running errands. The slant 4 was indeed either 152 or 196 (think half of a 345 or 392) and was functionally half of either of the mentioned v8's, but with a truncated block. Heads, distributors (except for the rub-block) and exhaust manifolds were all shared with the v8's. Cams may even have been the same. Mileage was decent, about 16-18 mpg. Most were 3 speeds, but I've seen 2 t98 4 speeds. There were few made, but IHC was willing to do one-offs, so the A-900 kind of gets lost in the sea of 'rare' IHC's.
If this one's solid, by all means do what you can to protect it as a rare vehicle. The rust-bug has the same zest for A-900 sheetmetal as it does for any other IHC of the era. If you can keep it clean and dry and kill the rustbugs as they show, it should be a good rig around town.
All in all, good find, and good luck with this project!