There were two commonly used border crossings between Bulgaria and Romania.Now the river Danube forms the natural border between these two countries,so you had to get over the water one way or another.
There was this one,a ferry boat,of sorts,that sailed across the Danube between Vidin and Calafat.
If you have “Flash Earth”,you can get a good view of this at:
43 deg.59’ 55.0"N
22 deg.56’ 0.0" E
The other crossing,of which unfortunately I have no pics,is a very long,very high road/rail bridge between Ruse and Giurgiu.An aerial view of this bridge can be found at:
43 deg. 53’ 14.0" N
26 deg. 0’ 25.5" E