I've fooled around with Annapolis Wherry sliding seats and riggers a bit so I'll pontificate a bit here. You shouldn't be afraid to challenge the orthodoxy of the Piantedosi drop-in. True, it's usually available and it is straightforward to install, but if you look at it closely you'll see that it was actually designed for a particular kind of boat - a recreational shell or similar. To fit into those skinny boats the unit is narrow with a 20" wide rigger yoke and this entails certain compromises.

Let's daydream a minute about the rigger that you could fit in your 38" wide wherry. First of all, why not spread your feet a bit further apart and splayed out?  That's the natural posture for Homo sapiens. I made up custom footstretcher brackets for a more relaxed foot spread that fit into the Piantedosi monorail. They work great but you can't use them with the full Piantedosi yoke-rigger because it's too narrow. (I built a custom rigger, but that's another story).

And then there's the seat. Whenever I went rowing for more than an hour, I got pains in the posterior that pretty much ended the fun. So one day I borrowed  a caned seat from my old  canoe, bolted it onto the seat-trolley and voila! I could row to exhaustion without seat pain. I could only wonder why no one had done this before?

I found out when I swapped the cane seat for the carbon-fiber hard seat in my 20" wide rescue Oxford Shell. It went from pretty tippy (I'm no sculler) to uncontrollable. I theorize that because 20" wide boats are so precariously balanced, it's critical that you stay precisely centered. If you're off by as much as an inch side-to-side that slight pitch gets is magnified by 10' long sculls. One blade goes up, the other stays down, arrrgh! So little hard seats (sculpted to let you feel when you're seated exactly on-center) are essential if you have a skinny boat, but if you have a wider boat - and you prefer pleasure to pain- a broader and more accomodating seat is the way to go.

While I made up a 'relaxed' rigger for my AW by progressively modifying a Piantedosi, you'd do much better starting with an Angus kit which is wood-based and so easier to modify. (Notice that one-half the cost for the Angus kit is for the $150 hard carbon-fiber seat. A webbed ash canoe seat is ~$50.)  It's your boat so I wouldn't worry about making modifications. And remember that the perfect boat is always the one with the rower it it. 

 Pontification is okay,  I appreciate your insight.  Thanks.  I've emailed back and forth with Colin Angus and have decided to go with his system.  In part the flexibility you've described played a part in it. 

   Yes, I built Colin's system for our wherry double, works very well. You'll need to mount a couple of blocks on the inwhales for fixing the rigger, otherwise no further modifications needed. Colin has lots of pics of mine.

   Should also say that Colin Angus solution is a lot lighter than the Piantedosi one.

   I'm thinking, also , that the Angus is much more stable too ! I do wonder how in the " build class " , which I will be doing in October 2023 , how do you build the Wherry to anticipate an Angus fit , both for tandem , and then single row at times ?

would you have any photos of that ? If so , shoot me some with details, please. Thanks a heap !