There are two ways to connect US 40 and US 6 in eastern/central Utah.
From Duchesne:
1) US 191 south to Kyune Pass Road to US 6 to Spanish Fork
US 191 is very remote with a high divide crossing.
Watch for Kyune Pass Road (paved)
US 6 has heavy traffic and is dangerous but has been significantly improved in last 10 years.
https://ridewithgps.com/routes/29352684
2) US 40 west to Heber City with possible Daniel cutoff to US 189 to Provo.
US 40 has moderate traffic with shoulders and lower crossing, some services
US 189 has narrow section around Deer Cr. Res. then 4 lanes then paved bike trail into Provo.
Lots of urban riding thru Provo down to Spanish Fork
https://ridewithgps.com/routes/6863901
3?) Dirt section between Strawberry Res. and Diamond Fork
Take FR 131 (paved) on west side of reservoir - Marina with store
Take FR 652 becomes FR 715 on west side of divide
Take Diamond Fork Rd to US 6
https://ridewithgps.com/routes/29938211
Nicest ride but a little rugged.
* From Santaquin you can take either US 6 or Old 91 / Hwy 132.
Nephi has way more services than Eureka, UT.
Eastbound riders are usually jonesing for services - westbound, less so.