I've had great service from a Williams foolproof on my 444 as pictured above. The 5D is a similar sight that is not screw adjustable, just slide and lock. It is cheaper. The WGRS, Williams Guide Receiver Sight, sits up top and is only slide adjustable.
If choosing the top mount I would go Skinner. I have used two WGRS' and they are fussy to adjust. The FP is a nice sight and easily adjustable if side mount suits you.