after the rot encounered on our 65, I'm not a fan of undercoating at all...the stuff gets loose and rusts behind, and till it falls off, no idea...paint is nice- looks good, easy to rinse off, and any rust under will bubble up so you can at least know of a problem before its gone- but bedliner i would *think* would be by far the best/most permanent solution- only worry would be prep needs to be proper or it could get the old undercoat type rust under it...I'd be tempted to put bedliner on a year or two after painting, just to be sure the base is rust free- gotta scotchbrite a bit for the liner to biite anyway. Some of the bedliner products harden as smooth as paint so again, easy to rinse off unlike the rough/cratered look of many undercoats.
when I got my 06 mustang, tore it apart while it had still never been wet/dirty and injected paint into all the vertical pinch welds(rocker bottoms etc) to seal the tiny gaps between spotwelds- when it started oozing thru, just taped over the bottom seam to hold it in...
sealed all but the lowermost drains off, and on the front frame horns (for lact of a better term) up behind the headlights, made little reverse 'scoops' to divert road spray from being able to blow into the frame- yet still alllow breathing and a exit point for flushing out.
scuffed the painted areas like wheelwell lips/gaps between front lower a-arm plates and used rollon bedliner over them to seal and prevent stone chips from starting rust. mines got almost 30k on it, over 3 yrs old, still like new underneath...my wifes 07has 10k on it, and looks considerably worse underneath- the front a-arms have rust lines around the welds/seams, some little rust showing here and there...we got hers slightly used, so couldnt treat it like mine for fear of sealing dirt/moisture inside...anyway, the week I spent under mine now makes me feel it was well worth the effort...the 09 still hasnt been on the road, so its probably gonna get the same treatment eventually before driving it, but for now its staying a 3 mile virgin
Our 65 I used POR15, and overcoated with their light blue 'tie coat primer' that must be used before topcoating- after getting it done, liked how 'bright' it looks when working under, so left it without topcoat...just painted the trunk floor/rocker areas enough it appears black unless you are underneath it. So far the paint is holding up excellent, but there are some areas where I used too thick a coat of seam sealer and as it shrunk the paint got cracks...other than that it could pass for 6 months old not almost 6 years