Hi Jay. I have cycled a few times in that area and driven through more often. Belfast to the Whites is actually rather easy terrain, dominated by Maine's lakes and rivers. Very pretty riding. You will seldom encounter a hill of more than 200'. Your only significant climb comes at the end (of course). Crawford Notch is 620' and quite steep near the top, but its scenery will distract you.
Autos would take ME-3 and US-302. ME-3 is not very good cycling and US-302 is downright poor. If I were riding it, I would take secondary roads, aiming to cross the Kennebec at Waterville and stay north of US-302 until the NH state line at Fryeburg.
I would take US-302 and NH-113 into Conway, then the West Side Rd north to rejoin US-302 in Bartlett. Visit Conway and North Conway only if you like heavy traffic and outlet stores. The remainder of US-302 in NH has moderate traffic but excellent shoulders. Expect lots of seniors driving big RVs.