shifting problem

i have 68 barracuda with a 833 4 speed (72 vintage) with 360/380 hp crate engine.it seems under hard acceleration i can not shift into second gear.the clutch pedal works fine no mis aligment.when the rpm comes down it goes into gear very easy.so casual driving is fine but taking rpm to maybe 3000 it will not go into gear. help!!!!!!
terry