Transmission question 1977 chevy transmission?

[dan p]

I have a 1977 Chevy Blazer, (Chalet model) It has a 350 engine and a 4 speed manual transmission. It will not shift down into second unless going under 2 miles per hour and many times unless stopped. Makes a grinding noise when try to shift down into 1st gear unless stopped. I have 2 clutches put in it in 1 year. The second time a week ago, a clutch and a used tranny put in. That tranny is doing exact same thing. So now what? Anyone else had these problems. Im assuming i should be able to shift down into second and first gear when moving. Any suggestions. thanks