IF you go with a 307/350 Olds V8 engine, you will need to obtain Olds frame mounts. G-body mounts preferably. AND you need to ensure to put them in the right spot if you do. On the rear of the frame you put the "long" side up, and the bottom bolt goes in the last hole rearward and the top bolt goes into the SECOND TO THE LAST HOLE REARWARD. They should be facing pretty much straight up and down. The 307/350 would drop right into place after that. Now, if you go down this route, you'd have to decide whether to stay on the computer or not. If you do, you'll need to get a V8 CCC box. Perhaps you may need other connections as well, I don't know. I've never converted a V6 to V8 with CCC. May be easier to get a NON-CCC HEI and carburetor and deal with it that way. There's other considerations like carb linkage to cables and trans cable hookups, etc., that will need dealing with too. Non-lockup torque converter? I don't know what an 84 V6 comes with. But it can be figured out.
Frame mounts look like this installed.