FWIW I have been having similar hot start issues with the battery in the trunk.
I tried the ford solenoid, the starter heat shield, grounded both the the battery and the engine block to the frame etc.
After trying the above with no success I finally figured out that if you disconnected the HEI wire and just cranked it a few times the starter was almost too hot to touch even with the engine cold.. = ground issue!!
I did a lot of reading here and on team chevelle and saw several posts saying that grounding the battery to the frame was not sufficient b/c the frame is relatively flimsy metal etc. People were citing factory assembly manuals which called for battery to block no exceptions.
What seems to have solved it for me was running a 2ga ground from the battery in the trunk to the engine block itself (not the bellhousing which is usually aluminum and doesnt conduct well). I also used a separate 2ga ground from a different point on the the block to the frame, a third 2ga ground from the body to the frame and an 8ga ground from the ford remote solenoid to the negative on the battery. Make sure all paint surfaces are clean, bare metal at the connection point - use a dremel or 80 grit sand paper to get to clean, shiny metal.
Hope this helps save someone some frustration in the future..