351 Cleveland popping and ticking sound

Need some help. I have a Ford 351 Cleveland. The problem that started my investigation was a popping and ticking sound out of the drivers side exhaust. The popping sound only occurs when I am at steady speed. It does not pop when accelerating or deceleration. I replaced the exhaust header gasket. I cleaned and sealed the exhaust slip pipes. I checked the following: timing @ 34 degrees, vacuum 17 to 18 Hg and holding steady. Checking the compression it seems to have low compression. The engine starts and runs fine. No vibration, no smoke. Any ideas?

Compression
#1 = 90 #5 = 92
#2 = 92 #6 = 70
#3 = 100 #7 = 92
#4 = 100 #8 = 90