The engine stands/perches for a 250 are the same as a small block, and they make small block to LS adapters so I'd go that route. It's best to keep the engine as far back as possible, trucks are already nose heavy. If you're adding AC to the truck the 5.3 A/C compressor is variable displacement and isn't compatible with Vintage Air or similar unit so it's not worth trying to make it fit in the stock location. I used brackets for a Sanden compressor from Qwik Performance when I added AC to a truck I 5.3 swapped, pn K10163.
Engine mount adapter plates:
https://www.speedwaymotors.com/ICT-B...ne,381861.html
Compressor brackets.
http://www.kwikperf.com/lsx_ac.html