Richie,
I'm almost certain that broadside date is wrong. The style and imprint are much earlier than 1776. 1676 would be much more in keeping. It is obviously printed in London and if it predates 1726 that's Fowler's theory out of the window. Would it be a lot of bother for you to get someone to decipher fully the imprint? I could then try to give a more precise date on it.

The imprint states as far as I can see 'Sold at the Bible and Harp/Heart in ............. Many printing houses in the 17thc were attached to inns and the imprints 'at the Bible and .......' are very common then. Not so common after 1700. However as a rough guide white letter had almost taken over from black letter by 1700. Unfortunately this can only be used as a rough guide and white letter was being used even early in the 17thc.