"If the bowl had been stronger, my song had been longer."

Solution 1:

The sense is counterfactual past conditional. The poem expresses this sense using a tense combination which is no longer standard:

"if (past perfect subjunctive), [then] (past perfect subjunctive)"
if the bowl had been stronger, my song had been longer

In modern English, this sense is normally expressed by

"if (past perfect   subjunctive), [then] (conditional perfect)"
if the bowl had been stronger, my song would have been longer

or in some dialects

"if (conditional perfect), [then] (conditional perfect)"
if the bowl would have been stronger, my song would have been longer