Any norm-referenced achievement instrument will have a straightforward spelling test on it: WJIV, WIAT-III, KTEA-3. You might also want a spelling of sounds-type subtest, such as is on the WJIV and WIST (the assessment used for spelling in the Wilson OG-based remediation program for reading and spelling). The WIST-associated WADE could also be useful in looking at phoneme knowledge.

You may also consider other phonological processing assessments, such as the CTOPP-2.

...pronounced like the long vowel and first letter of the alphabet...