These questions are a bit hard to answer - as the overall component package is not known (on this site).
Example: running original light-weight non-adjustable lifters gives you a LOT lighter valve train than a new set of Isky solid adjustable lifters . . . with Johnson's somewhere in the middle. The valves can also make a big difference - size, material, stem diameter, head design, etc..
Also, what is 'max rpm?' . . . how far are you willing to spin the motor before it either lets go or you let go of the throttle? What rods are you running, what oiling system, what bearing clearances, whose crankshaft . . . steel main caps???? You get the point . . .
I would probably shoot for 70 - 75 lbs on the seat for a higher rpm street build with hollow adjustable lifters - my guess is that you'd get to 5500 rpm quite easily (not knowing the weights of everything in the total package).
Also, consider that most flatheads that I've seen don't have the necessary ignition system for higher rpms, don't have ports big enough to flow the air/fuel necessary to make much HP, etc..
Now - if we all could have personal engine dynos and unlimited time and money - would make this a lot easier to ponder. LOL