There is no difference to the ㅆ을 거예요 structure, because the verb before 거예요 is put in the psst tense. The difference comes into play before ㅆ, in the form of whether 아/어 is added or not.
(by vowel harmonization, 아 is added after a syllable containing ㅗ or ㅏ, and 어 after all others).
- 먹다 -> 먹었을 거예요. (어 and ㅆadded after 먹)
- 찾다 -> 찾았을 거예요. (아 and ㅆ added after 찾)
- 가다 -> 갔을 거예요, (no 아/어 but just ㅆ after 가)
- 지나다 -> 지났을 거예요. (similarly just ㅆ after 나)