I'm pretty sure (read: not sure at all) that Mass in F Minor marks the first Electric Prunes which saw David Axelrod oversee production, most of the composition, etc. It's also around the same time that the group went through a major shift in line-up (around the same time Kenny Loggins joined the group). What's also noteworthy is the Prunes couldn't finish the record, and they had to farm it out to a Vancouver band, the Collectors.
In any case, Electric Prunes + Axelrod + faux religious mass = go nuts. Here's a smattering from the album.
"Kyrie Eleison" (this probably has the most obvious Axelrod touches to it)