I'm sure that your home made tool works far better than the NAPA tool you show in your post. I never could get one of those tools to work consistently. I have tried them all, and the only one that works easily and consistently for me is the one on the right made by Lisle. Unlike the others, it is obvious when you have the tool positioned correctly, and it works from the rear of the handle, not by going under the handle from the open end.I'm pretty sure you have the u-clip retainers. They had them on the 64 Merc for sure.
NAPA (Canada) UltraPro #60371
Handle Removal Tool, Removes Spring Clips
If you have trouble finding this style of tool, you could do what we did 35 years ago ... improvise! (We were living in the middle of nowhere!)
I used a hacksaw blade, ground all the teeth off of it so that it slid through without marking the panel. I used tin snips to cut a "v" notch in the side of the blade to the mounting hole, and then bent the resulting hook slightly sideways. I then slid it in from the head end of the clip, hooked it, and pulled it off.