318 rpm drops to 400 in gear WHY???

I ran my 318 with similar components (same cam) in a 68 cuda using stock 340 converter and had no idle issues (once I solved the fuel loading problems). Only other difference was I used rhoades lifters... they worked pretty well to help idel quality by bleeding off at idle and reducing cam profile but coming back in quickly when rpms increased.

If you are using the larger 318 converter it may not be enough but stock 340 or better should work (of course "should" means "no gurantees")