Hi David,
I’d like to suggest an alternative approach. If I were you, I’d be looking for the following: a mast step that works, stays in place, doesn’t rot, and one that doesn’t permanently alter or damage the canoe. Given these wants, I’d do this differently. Lateral forces are the concern, and that’s precisely why screws should be great. Sure you could rip a screw out of cedar by yanking on it or the step, but that’s not what the mast is doing. Heck, nails resist lateral forces- that’s their job (but use screws). Screws would work even better if they ran from the planking through the ribs into the step (i.e., from the outside of the canoe)- any chance you’re re-canvassing the canoe?
I personally wouldn’t use epoxy either. A step of any significant footprint will likely be permanently bonded to the canoe by epoxy. If you needed/wanted to remove it, it might be very difficult because a heat gun likely won’t de-bond anything but the edges of the step- wood is an excellent thermal insulator, so epoxy at the center of the step probably won’t be affected by the heat gun.
Ash and epoxy? If you use epoxy and then use ash for good bond, you’re inviting rot down the road (ash rots beautifully). Oh, right, you can seal the ash well before epoxying it in place… but then you don’t need ash because the epoxy bond is no longer to the wood- it’s to varnish!
There are untold numbers of Old Towns out there with white oak steps, not epoxied in place, but rather screwed on from the outside of the canoe. When I’ve removed many of these, they appear to have little or no varnish on the bottom of the step, yet they’re not rotten, they’re not permanently bonded, and they’ve never ripped out. So if I were doing this, I wouldn’t reinvent the wheel, but do it as it was done before. If the canvas were good, I’d apply a pre-varnished oak step from the inside using screws and perhaps a non-epoxy glue (like Titebond III; though I don’t know how well Titebond bonds to varnish), knowing that it might need to be chiseled out if you ever wanted to remove it.