to convert the motor to a carb you have a few options-
1. buy an intake that will fit the magnum heads vertical intake bolts. edelbrock makes one and also a foreign version made by crosswind.
2. you can make a jig(I made one) or send out your heads to have them redrilled so an LA intake can be used.
the next thing that needs to be addressed is the problem of getting fuel to the carb. the magnum motor cam has no place for the fuel pump eccentric to fit and the timing cover has no where to bolt the fuel pump to.
1. use the LA timing cover/water pump/pulleys/alternator(will need spacers/washers to put it in alignment with the pulleys)/power steering pump. AND either an adapter(you can but these) to use an LA mechanical fuel pump or use an LA roller cam that has the long snout to fit the eccentric on to.
2. use an electric fuel pump.
my thoughts are, you already have the motor use it! get an 88-90 a500(light duty) or a518(HD) AOD trans. these are the easiest years to make the OD work. and you will have a great cruiser.
pic of jig I made to redrill magnum heads to fit an LA intake. you can see the 2 different angles of the bolts-