In order to put the 305 into the '87 several items need changing. The current '87 trans will not bolt up to the V8, need to get a '82 - '92 700R4 from a Camaro or Firebird. There is a torque arm mount on the transmission.
The frame mounts need to be changed for V8 ones.
The brake line that goes across the K-member needs to be re-located, else the frame mounts will crush it.
If the '87 has a tach that needs changing to a V8 unit.
The current driveshaft is the correct length, no differnce between V6 & V8 there.
W/O the ECM another method of TCC lockup will be required. Too many options to list here. Unless of course a stick tranny is used instead.