there is merit in mounting the radiator back in the bay as you suggested. easy with the ke/ae radiator as it is side mounted to the radiator support so would be easily spaced.
i'm actually doing the same thing in my AE86. the radiator will be mounted as far back as possible to free up space for cooler pipes.
the distance between the firewall and radiator support is greater in a 70/71 than it is in an 86, the 86 has a deeper nose though so there is plentiful space for an intercooler and little room behind a 70/71 (moreso 70)
heres a pic of my old 16v KE which shows the distance between radiator and front pulleys, not a ZE but the distance will be the same. obviously run a thermo to maximize the gap
