Does your block have stock hemi mounts or 440 style mounts? You need to see what it has on it first. The aftermarket blocks I believe can be had with different mounts or no mounts.

When you go to a blown hemi, the required exhaust tube size is usually 2-1/8 at a min, and fitting those size tubes around stock mounts is not a fun task. My 2-1/4 tubes drop right out. My engine would be centered completly but the headers hit, so its still slightly offset. Don't forget the external oil line that has to run through there as well. Things get a little tight with big headers.

Maybe somebody on here has fit big tube headers with all the above, but its probably not fun. Mine can be put in or removed fairly easily without disassembling anything. I would go with a plate. Besides, what else are you going to mount the alternator to?