F0426 works out to a Flint produced engine built on April 26. The L code is bit tricker, as it varies depending on year. What I have found by checking the codes at the GM Heritage site, is that it is not a passneger engine, but rather a HD truck engine. If it is a '58 engine as you suspect, the you're in for a bonus, as the L code is for a 261 from a 6000 series truck, rather than a 235.
