Don,
There really isn't much of a difference between the 2 lengths from what I've seen as far as KE requirements.......I'm afraid you are still way lower than what I would consider safe for that size cut, and even that STYLE (swing open) for that matter.
I'd even be leary of you using the small diameter Rocket Steelhead, or even the Snyper.

1 1/2" is still a large diameter mechanical and I would lump it in the same category as the 1 3/4" version......it takes some whoop to get these heads open and keep that surface area moving thru the target,and you truly don't have enough to do both.

Personally I wouldn't let someone out the door with your set-up and those heads if they were hunting with me.......that's how little faith I would have in their effectiveness on every shot.
Frankly I wouldn't even put you in the "Marginal" end for that particular head, you are (IMO) BELOW marginal for ANY mechanical head.......you'll be MUCH better served with a quality fixed head.

Welcome to the board and we'll look forward to having you around!