Sylvain, the Mater I used to convert to a Slot Car, is probably close to 1/64 scale, except it has a Cartoonish TALL Cab height, but the rest of the proportions are 1/64. But I do know the large 1/55'ish Mater you speak of, and some folk have used that one to convert, and used the 4 Gear/Specialty chassis in doing so, but I think that 1/55th scale is too much of a stretch to go with other HO scale stuff.
BTW- the small Mater I used was a Give-away toy by Kelloggs Cereal about 10 years ago ? It was never sold apart from the promotion, as many others have been. Here's a little Caveat, about the Kelloggs Mater, whatever Plastic was used, is now very brittle, I dunno if it was brittle from the start, or became brittle with the passage of 10 years, but it can shatter pretty easily if dropping on a hard floor or takes a good Wall-shot ! I've partly remedied this situation, by *painting the entire under/inside of body with Testors Liquid cement #3502 , that's the same stuff that we used to chemically MELT Aurora Plastic, to make the colored Goop we use to restore Aurora Bodies with. Use with Caution though, as it does contain MEK and the Vapor if harmful.
PS- a while back, I bought what I though was a plastic 1/64 scale Mater to convert, but it turned out to be highly detailed RUBBER ! Anyway, I carved that rubber body up somewhat, to make a more realistic Tow/Push truck, which might be found around an old Dirttrack, here is the result... :D
