So, BID they shifted D-Darts at 7000+ and were turning 8000+ in the traps, but that's drag racing only, and not high mileage hoonage, the stock rods and heavy-ish 10.5:1 pop-ups were the limiting factor; the the forged 273 crank is reputed to be good for regular use spinning to 8300 max, but the rest of the package would need significant upgrades to last going there.
I'm actually eyeing a 300.5Cid 273 build myself as I have a small bearing 3.58" stroke SCAT 4340 crank for my '66 Formula S' OEM 273 Commando. Pistons will be custom and I'll be using K1 rods and a Hughes Engines main stud girdle. In my case I'm aiming for a streetable peak HP RPM of 5500-6000, and a safe(ish) rev-out (for autocross) to 7000 (valve float not before 7500). I'm aiming for 10.5:1 compression, which may require dished pistons with that stroke. Heads will be ported 273 or "...302" head castings with 1.88" and 1.6" valves. I'll be running an LD4B intake with 1/2" partially open spacer, a 650-750 QFT Circle Track carb, and Doug's headers. I have a set of ported 273 heads already, not sure if one is good or not (stuck valve); their flow was nothing to write home about but they did support 317 WHp @ 5809/314 FtLb @ 4785 on a 0.030" over 318 at about 9.75:1 with a fairly hot Hughes cam (Max Velocity mechanical cam, part number: HEV4550AS.).
I have not gotten to the point of configuring pistons yet, but they will be custom ordered from one of my builder's sources (Diamond, Wiseco, JP, a couple of others...) The cam will likely be a custom mech roller from Comp.