While I am no expert on engine conversions, I hope I can offer some advice that you'll find handy.
70s-82 302s were Clevelands. Slightly different dimensions to the 90s efi Windsor 302.
I'm positive that any engine that you plan to put in your XE other than a replacement 200/250 would require new engine mounts.
Clevelands that are older than your XE would be more difficult to get an engineer's certificate for, unless they run on straight gas. I'm assuming that an efi 5.0 would be more difficult to install due to the engine computer, but I'm not too familiar with how the engine bay communicates with the rest of the car, if u get me.
Regarding having a shaker or scoop or whatever, I'm not too sure about those with LPG cars. I remember a brother of a friend of mine though with a HZ on gas bought a scoop for his bonnet, but the engine would always stall whenever he tried to take off. It turned out that the air entering the scoop would blow out the gas going thru the carby (or something like this), starving the engine! He then put a towel over the hole to stop the airflow. The towel caught fire...
Hope some of that helped in a small way. A mate of mine did a home-job conversion for his XD ute: 250 column shift turned into pre-pollution 302 (converted to LPG by himself) with a 4 on the floor. Did everything himself. So it is doable by yourself, although I wouldn't recommend "his" way... if he gets busted for it he'll cop massive fines due to it all being illegal! Ultimately do your homework first.
I hope u get the XE up & going as a tuff kruiznwagon