I have this same problem with my TH350. With the 350, is it the vacuum modulator that could be to blame? I can't kick into passing gear once I reach the last gear unless I slow way down or stop. I think the guy that had this car before me abused the tranny.
67 Coupe 327 LF7 - Trim tag revealed that it was one of the first 1,000 Camaros off Van Nuys in 67 after I bought it!
Edelbrock performer package, ported 305 heads w/ 1.94/1.5 valves, screw in studs, summit 1.5 roller rockers, TH350, 10 bolt Pertronix ignition, Hotchkis suspension & sway bar, KYB's, Dynomax headers and exhaust, K&N, Cowl hood