Comp lists 5/16" pushrods for '72 to '78 and 3/8" for '69 to '71. Since they list the same lifter for all years, I would think the difference is in the heads, not the block. So, if the blocks are the same, order 3/8" pushrods.

The first thing I would do though, is to order 1 part# 7703-1 Hi-Tech checking pushrod to check your geometry on each corner of the motor. Then order pushrods that are closest to the length you find by checking.
http://www.compcams.com/technical/Ca...106-07/249.pdf
http://www.compcams.com/technical/Ca...106-07/264.pdf
http://www.compcams.com/technical/Ca...106-07/265.pdf