kcassells and Redtruck12 have good ideas! Practice getting the transom in and setting the clamps, keeping in mind that it always is bit trickier to work with thickened resin on several surfaces. Having an extra set of hands (or two) really helps; ideally, you'd like to have at least one person inside the boat and one outside.
When I did mine, I mixed a quart or more of PB initially, and I premeasured everything needed for a second batch. I also made sure I didn't mix the resin too hot, so it kicked a bit more slowly to give me more time to work. I also had a pair of calipers handy, to measure the thickness of the transom as I clamped down (which helped me make sure I kept the thickness uniform and within Mercruiser's specs).
Good luck!