I understand. The 4.3 was good but was anemic in a full size truck.
The evap should take care of itself, I would just put the hose from the purge valve inlet to the air cleaner,dont hook it to a direct vacuum source, just vent it into the air cleaner area,dont use the purge valve, just use a one way check valve, to allow pressure to go towards the carb from the tank.
A 383 should wake that turtle up into the hare it should be.
you will need to get a controller to run the 4l60E transmission though.
Like I said , it wont be easy.Or did you change it to an older 700 r ?