the pros all say to run thick cables up to the engine (starter and block)

you can get away with earthing the body, if you also connect the block to the body, for a while. It will be difficult to diagnose when problems occur

1200s only have a small gauge wire grounding the body (suitable for 35A draw). The high current the starter draws goes through the negative cable to the engine block. If instead you fasten the Neg cable to the body and run a cable from the body to the block, it will work, but is susceptible to earth problems when the inevitable slight corrosion occurs. The 1200 has very few starter connection problems even after 40 years because they run the cable directly, and it is still recommended. Just think of all the lighting issues that have been caused by body grounding, then imagine that happening with the starter