The OD from the three speed will not bolt up to your 4 speed. If you want a HD overdrive I would suggest the gear vendors as long as you have some $$$ hanging around. Think they start around 1K.
Grant 1950 Chevy 3100 1952 GMC 150 (In Pieces) 1968 Chevy K20 1966 Shovelhead 1978 BMW 100S