Cheap easy way to lower your RPM,s for now.
Use a 29-30" tall tire in the rear.
IIRC,,I used a 235 75 15 on my 65 Malibu SS that had 355's in the back,,made a big difference over the 26" tall tire.
Looked a little funky,but I did not care @ the time,I was driving on the freeways alot.
MBHD