You said the 116 is an '84. That'd be aluminum. The 117 at '76 was cast iron.

As I said, cast and aluminum heads are NOT interchangeable!

3.5's and 4.5's have different pistons. 4.5 pistons are flat and don't reach the deck of the block. 3.5 pistons do, and are recessed for the valves on some and have small center cutouts on the others. The major difference is the crank - a 4.5's crank has longer travel to cover the longer block.

A m117 crank will not work in a m116 block because the pistons would be pushing out WAY too far above the deck.

If you have an iron-block m117 with no heads and an iron block 3.5 with good heads, do the swap. A lot of people think the 3.5 has more power, but it's the way it's geared and actually has less torque, though about the same HP. If you put 3.5 heads on a 4.5, you get more power than US 560's had stock I believe.