What diameter track do I need

What diameter track do I need to run parallel to a quarter circle of O72 track. 

My lionel big boy has 2" overhang and a long propane tank car has 1 3/8" on a tubular O72 curve. So the tracks must be separated by at least 3 3/8", say 3 6/8"(propane less for larger radius).  Should I use a  O84, O96 or 108 curve to have tracks parallel?

Second, where can I acquire a half circle of tubular track in this diameter??

Third, assuming I cannot find this, how long a straight should I put between each section of O72 track to approximate this diameter?

thanks for help

stan