Trans shifts hard into gear?

put a newer 350 turbo in my 1963 chevy pickup which has a 350ci engine stock not tuned in any way. and the idle runs super high now, i had a manual trans before i swapped it for the 350 turbo. and when i put it into gear i dont understand why but perhaps since its so high an idle it slams into reverse BAD but when driving and at stops and starts in gear no slamming shifts reeeeeal smooth. and it slams into drive too but not as bad as reverse does. any ideas why???