I’m building one myself this winter, not for racing I’ll be keeping the Pearson for that.
I have a choice of some old steel 531 frames that would have run 27×11/4″ wheels. They had shed loads of clearance anyway, but with 700cs there’s even more.
The canti bosses are coming from Ceeway, but I’m waiting to see which frame I decide on so I get the right ones.
The good thing is that old frames had long semi horizontal drop outs, so can be converted to singlespeed without any mods.
You won’t have the raised bottom bracket, but that’s not a major issue.
Add thirty quid for a powder coat, and for well under a ton you’ve got a funky singlespeed crosser.
crosser-singlespeed-niche-retro-all-in-one
Edit; as the older frames were 120mm you can directly fit track or flip flop hubs as well.