The 2 barrel 289 was rated at 200 horsepower at the flywheel. The 4 barrel 289 was rated at 225 at the flywheel.
Max RPM would be 5,000 to 5,500. The stock cam really stops giving any more power at 5,000 rpm. The stock hydraulic lifters will "float" at anything above 5,500 and you could risk valve to piston contact. Very dangerous!
I would not rev it past 5,000 myself. Risks breakage and would not really go any faster.
Sounds like you have got the exhaust side opened up. Want any suggestions on your next steps to go faster? Are they long tube full length headers? I have built many a small block.
Rob