A/C on big block

[burb1085]

I don,t have any pis of the a/c compressor right now, but I know the clutch on it is burnt out(it locks up when I turn it on). Basically my questions are this, what compressor goes on an 87 big block? When I look it up they give me a choice of two, a Da6 or an Hr4 I think I got it right, but both of them look almost exactly alike. I'm not sure if the p.o. put the wrong one on there, and the one thats on it is re-manufactured. Could I use either one? Also wanted to know if I could switch to the 89-91 pulley set up and use the smaller shorter compressor. I have an 87 R20 burban with a big block.