Valvetrain noise - Magnum headed LA 360

I recently put together a Magnum headed 360 for my Dart -
bottom end
Summit 6901 hydraulic cam - 0.470" lift (with 1.6 ratio rockers) 218 intake and 228 exhaust duration at 0.05, 276 intake and 286 exhaust advertised duration
Comp Cams lifters with pushrod oiling provision
Mopar Performance conversion pushrods for Magnum heads on LA motors
Mopar Performance thin head gaskets
stock Magnum heads redrilled for LA manifold bolt pattern
Mopar Performance valvesprings with 2.2l retainers
Edelbrock LD340 intake with Carter AVS carb

It runs pretty good but the valvetrain is noisy, not really audible at idle but becoming noticeable at about 1500 rpm and getting progressively noisier - by 4000 rpm the clatter is pretty loud. The motor revs cleanly round to 5500 and will go to 6000 but doesn't seem that enthusiastic about it.

I didn't measure lifter pre load when I assembled the motor but it looked like there was quite a lot of preload - possibly too much. Could too much preload cause valvetrain noise?

Since putting the engine together I've seen reference to blocking off the oilways for the rocker shafts when using Magnum heads - I just bolted the heads on. Doesn't this block the passages sufficiently or is something else required?

thanks