Valve seals tore on break in

Pull the heads and take them to a machine shop. That would be the only correct fix. Your head needs to be machined for the larger lift cam and the retainers you are using. The retainers squashed the seals. Common problem that many do.

Clearancing things is part of an engine build . You my friend skipped a critical step. What else was skipped over? That would be your next question to asked yourself.

Like did you seat the thrust bearing. Did you check the rotation torque. All critical for engine building plus many more procedures to make them last for many miles. Did you clearance the Cam bearings ? Many don't. Things you should be asking yourself.

The most critical is balancing. You don't know how many pro balanced rotating assemblies that come in that are over 80 grams out. Molnar stroker kit for a Ford FE Pro balanced? 128 grams out. Never trust anything you buy.