problems with kickdown shift RPM

I have run into a problem that I've never seen on a torqueflite in 40 years. I installed a freshly rebuild 904 behind my small block with an A&A standard pattern valve body (which I have in 2 other cars). Car works fine in drive and delivers crisp "drive around" shifts, but when I floor it at 10-20 mph it kicks down, but only holds 1st gear to about 3500 rpms. I have tried adjusting the linkage , but with no improvements. Does anybody have any suggestions as to what is up? My other transmissions are set to hold 1st until 4500 rpms and I never had a problem like this before...