Not sure where the foothills are but if you are on semi-flat ground you should be okay.New Holland says 60 pto hp on their 4X5 balers they make now.JD and Vermeer say 55 and 50 min. respectively.I pull 6X5 John Deere 535 with a77 hp Ford.That being said,I don't pull it as fast nor do I have steep hills even though I am in the mtns.
Another thing you can do is make the windrows smaller.Mine a re 1 and 1/2 to 2 9'swaths rakes together most of the timeThat way you'd know what it takes.I have pulled a 4.5X4.5 New holland chain bale with a 165 Massey. However they did not make as tight of a bale. Also,if the baler deal is right ,you could bale to 4' diameter and stop if 5' is too much.Would not be much different than a New holland 630.