It isn't always possible to determine a single "correct" pronunciation for a word, especially not for a word like this that is obscure and mostly archaic.

You referenced the Oxford English Dictionary in the question, but I think it actually has a good discussion of this in its entry for *heel, v.1*:

>*Modern spelling.*
 >
**The spelling of the word continues to vary considerably in modern use**, probably reflecting its limited currency. The spelling *heel* is very common only in sense 2c, where some folk-etymological association with heel v.3 seems likely.
 >
*Specific forms.*
>
In form *hool* perhaps showing an alteration by analogy with whole v. beside heal v.1
>
The origin of the forms ***hail* and *hale,*** attested in Masonic usage (compare sense 1) from the 18th cent., is unclear. The forms **may either originate in regional pronunciations** (compare Forms 1 β.), **or be due to** the influence of Older Scots **graphic forms** with -ei-, where the postvocalic -i- indicates vowel length (compare reveil v.).

The "long vowel" in modern English that usually corresponds to Old English "e" is /iː/, but as mentioned in the quoted passage, regional variation has existed.