B series in a 61-62 Valiant?

Whats the diff between 62 and 63 A bodies? Shumakers kit says 63-66. I suggested to my brother in law to put a Bigblock in his 62 Valiant. He was told hed have to hack up the bay and ditch the heater just to fit an LA motor. I was told they were made to fit a Wideblock poly 318. Any info is appriciated. Thanks