That is definitely an "I" between the 3 and the D, but I think that it was used in place of a 1 for some reason.......might have been at 4:55 on Friday.
I see what appears to be a power pack head, so my quess would be a 283. A '68 307 would have had heads with the accessory holes. Of course all of this is assuming the heads weren't changed at some point.