Dot 5 is what your truck would have had in it when in service, but your truck may have dot 3 in it now, when Dot 5 is new it is usually purple in colour, unfortunately as it ages it tends to turn yellowish which is the same as dot 3 the two types should not be mixed. If you take some dot 5 and add it to some water in a jar and shake it will not mix with the water, but dot 3 will mix. For loads more info check out steelsoldiers.com
You can also download all the technical manuals from there for free. Also there is a Facebook page called M35a2 deuce and a half which has lots of like minded people on it. Big mikes also sells a remote master cylinder reseviour which is a great investment, as you don't have to keep taking the top of your master cylinder to check your brakefluid level, you can mount it on the firewall or in the cab and easily check the level every time you run the truck. Don't forget to post some pictures.