I have cycled every possible paved route between the two parks and some dirt, too.
I have a tour on Crazyguy which is the route from Jackson to Yellowstone to Glacier - i.e. reversed.
This route roughly follows US 89 east of the divide.
It is much better than Hwy 87 west of the divide.
Less traffic, better views, better shoulders.
Plus, southbound, you should have more tailwinds than headwinds.
If you will be coming in from West Glacier, that will let you do Going to the Sun Road.
Best camping in Glacier - Avalanche on the west, Rising Sun on the east.
Apgar and St. Mary are huge, bleah campgrounds IHO.
You really should consider heading up to Many Glacier - awesome.
If you did that, you could take IR 464 to Browning rather than US 89.
The stretch of US 89 on the east side of the park is narrow with RVs.
Browning has a nice private campground on the west side of town.
There in-town camping in Dupuyer and Choteau.
They'll let you camp in Fairfield if you ask.
Great Falls has a nice riverside trail and bike shops.
The ride from Belt to Livingston is super - stunning vistas.
Lots of places to camp.
Take the back road into Livingston north of the river -
Then heading into Yellowstone - take the road on the east side.
Best - J