It should fit, might be necessary to change oil pan, and adaptor plate at rear of engine. One point to consider, the 706 used 2 different front frame rail designs. The 615 engine will be close to the early 706 with the C263, I am not sure how much if any block differnce there is between the C263 and the late 706's C291. If memory is correct the C263 and C291 are same block and crank with a larger bore on C291 so you should be within reason.