Is it possible to get a Cambodian visa on arrival for US citizens crossing overland from Savannakhet/Pakse to Phnom Penh? Have done it many times via Poi Pet but the Pakse route seems more of a remote area so not sure if that's possible or if the bus would wait for me to get a visa. Thanks.

Yes it is available at the Veun Kham crossing south of of Pakse.

It is also a Cambodia e visa crossing. https://www.evisa.gov.kh/information/port_entry/3

Map: https://goo.gl/maps/xwB7HA7X2av9Y3Zg8