Help with rpm drop when put in drive.

[grubsster]

Hi all. I recently installed my 283 engine and had a major issue with driveability. Contacted Comp cams, B&M and Summit. All recommended a 2,000 stall converter. I swapped in a B&M 20400 which is a 2k stall. My issue is better but I'm dropping from 900 rpm to right around 500 rpm in drive. I would prefer to set my idle around 800 and have my in gear idle around 600. Where should I start? Timing is @12 degrees initial which is what the engine builder recommended but I think since the cam is a mild performance cam I could go higher. Any help is appreciated.

283, vortec heads ,edelbrock 600 ,and rpm intake comp cam with a 218 duration@ .050 ,th350 ,3:08 gear on rear end 1966 c10 long bed, cam comes in at 1300 rpm.