If you divide the rod length by the stroke you will obtain the rod/stroke ratio
6.4 divided by 3.98 =1.608
6.7 divided by 3.98 =1.68
The higher the ratio number the better it will be at revving higher. The shorter rods tend to give a little more torque.I have a 406 chevy with 6" rods and 3.750 stroke = 1.6 and I happily rev it to 7500rpm and sometimes 8000rpm.
So it depends on what you want. The stock 400 small block with 5.575 rods has a ratio of 1.48 very bad for high revs ,but it even took 7000rpm regularly
Longer rods require special pistons, mine the piston pin is behind the
oil ring and works fine ...so far
Good luck