Actually none of them were ever hot sellers. They go together well and are decent kits but are very simplified by 1/24,1/25th scale standards as they were aimed at the youth market. But there is tons of room for super detailing.
Actually the allison v 12 is physicaly much larger than a detroit v 12. The detroit v 12 is a whisker under 14 litres the same width and height as an 8v71 and a little longer than a 6 71. As far as the allison goes Tyrone Malone put one mid engine a coe freightliner many years ago as a wheelstander. Last I knew it belongs to Gary Riese as well as the Super boss and other Malone stuff . Certainly would be a neat project.
There have been several small companies that have made big rig conversions for pu's of varying degrees. The Revell kit you mention dates back to the 70's as the sneaky pete kit. I've seen one's close the kit but not exact.