Have you checked the vacuum at idle with a vacuum gauge? It should be some steady number between 18-20 (in Hg?) at operating temperature. A lower reading, fluctuating reading or other anomaly could mean any number of things including a vacuum leak.

Does you car have an economy gauge (which is essentially a vacuum gauge)? Is it acting any differently after the head job?

You asked about what spray to use. I've used carb cleaner with no ill effect. Just not into the intake stream. Short bursts directed at the head-manifold interface should be fine. Keep the spray away from the plug wires to the extent you can.

Sixto
91 300SE
81 300SD