Thanks guys for the advice and pic.... the block is painted black, so difficult to read anything without a lot of light.

The date code looks to be D056 or D058. The teufert site lists a block code of 500557 (I thought I was seeing 300667) for late 1976. This site doesn't have an XS for '76, but at www.wallaceracing.com/engine.htm I found an XS engine code for late 1976 (170hp yay!!).

The www.spottsperformance.com/engineid.htm page identifies 500557 as a 400 Service Replacement engine.

So I'm now assuming I have a 76 engine in my 67 bird.

(As a guide to others, the block number is visible on the passenger side through the gap between rear of hood and firewall.... the date code at the back of the block is best seen from the driver's side.... both are on horizontal surfaces.)