never done that kind of swap, i went up not down myself, and it turned into a clusterf*** 350 to 454, unlike me youll probably be able to keep your transmission, but keep in mind what transmission you have, and what years the truck, the chip(or computer if you replace the whole thing) and the engine are.
its the little things that really eat at you like sensor connectors and accessories, like oh i have to go buy a new alternator because this one has a mounting hole in the center and the bracket has a mounting hole on the top instead, or some sensor switched from the square connector to the flat one. its not necessarily hard to deal with, but its something to keep in mind, there are a few places where i had to chop off the connector and run the wires to a different shaped plug but the wires were all the same its only the plugs that had changed, and the difference was only 2 years, engine was a 92 truck was a 94, but there were half a dozen different plugs
and the reason i say pay attention to the years and types of things like transmission, the new chip is gonna need to know what transmission you have, what diff, etc. you cant just buy a computer from a truck with the 305 that had a different transmission or gear ratio because then it wont shift right, truck year only really matters because it determines what wiring harness youve got, if you can match them perfectly, get a computer that matches the year and parts, engine that is the exact same year, you might be able to get away with just plug and play, but being a different engine i still cant guarantee that