Oil pan leak - wrong pan?

Have a '73 Duster with a '68 340 block. Have an oil leak at the back of the pan and we cannot get it sealed.:violent1: I originally ordered an oil pan gasket for a '68 340, but did not match what we took out, so ordered one for a '73 and it fit (we thought). Leaked oil even worse. So my mechanic and I are guessing that we need a '68 pan to get this thing fixed. 2 questions:

1) What year and c.i.d. engines would be the same pan as a '68 340?

2) Don't know what a 68 pan looks like....does anyone know if the '68 pan would have any clearance issues with the '73 headers, starter, crossmember etc?