I'm not positive what all would need to be swapped.
From reading about the 700 shaft, it stated it could be used in the 60E/65E with the use of a different oil seal on the front end.
Looking at the 65E shaft it looks like the TC/TorqueConverter would need to have a 65E type TCC/TorqueConverterClutch installed inside an older 700/60E type TC.
The 65E was used with the LS1 engine which uses different spacing & TCC etc... By the time you build a TC it may cost as much or more than the 300M shaft.
Maybe a 700R4 builder on here will know if anything else is needed.