Since a variety of readers bring different levels of experience and understanding to these threads perhaps some elaboration is needed.

Since most engines are attached to the frame through flexible mounts, just mounting an alternator to the (comparatively) non-moving frame will create a belt tensioning problem. The advantage of the units pictured earlier is the alternator moves with the engine. With a frame mounted alternator and a moving engine you'd need a somewhat complicated belt tensioning system, likely defeated by the limited space issue that started this discussion.

The primary reason the tractor sourcing came into play is the drive pulley being of a V belt configuration. Most (if not all) contemporary automobile configurations have a "flat" belt style pulley. Fine if that works with the rest of your system, not if you require a V belt. Of course you could get an alternator one place, and a pulley elsewhere.