Here's a good flathead video...

[Val406]

https://www.youtube.com/watch?v=sUvM5te8ZoY

[Mbowling]

Luv a good flathead.  I’ve got a pretty stout 286” I built in my 34pu..also a nice complete 40 engine & a nice37 60hp just for garage art.

[drdano]

Torque and horsepower curves look like horizontal lines.   

[gdaddy01]

at what rpm would you shift this thing ?

[cjshaker]

at what rpm would you shift this thing ?

No matter the power level, to keep things safe, you generally wouldn't want to push a flathead much over 4500. 5000 is about the max on a well built engine, a little higher back in the day when the engines were considered expendable and easily replaced. 3 main bearings and a less than ideal valvetrain really limit them.

[gdaddy01]

with the torque curve or line in this case , just seems you could shift about any rpm and be ok , yes up to 4500 .