Sealing Gaping Wound under the cab from manual tran change; PIC

[darkone]

hey guys, this is what i have goin on, pic shows pretty well whats going on. i think im going to have to fill the gap with something after i get that grease out of there. the problem is, there is a gap about a half inch wide all the way around that lip. what should i use to fill here guys? thanks! (metal is going to be prepped and painted before anything is applied to seal this problem area.