Barrel Depot has just released an aluminum and brass 90mm M48A3 barrel made specifically for the Tamiya kit and is item #35039. Runs about $15 and is very nice.
Converting the Tamiya M48A3 kit to an M48A5 will take a bit of work. Verlinden used to do a resin and PE set for the conversion and Lo Models did one as well. Many changes must occur starting with the gun, air cleaners, headlights and guards, and the cupola can be changed although there are photos of CAARNG M48A5s with the .50 cal machine gun cupola. Additionally the tracks should be updated to the T-142 type (AFV Club makes them).