gear box noise

hey all i am new here and a new 76 kombi owner. since buying it a loud wirring noise comes about in fourth. i got an opinion on it and its possibly the main bearing would this sound right? also just wondering what the best option would be, either just replace the bearing, or switch out the box with a second hand one then slowly pull apart the old one and rebuild it. but anyone know the roundabout cost of a second hand or rebuilt gearbox? and where to find one, im in vic sth east of melb. i have sourced one second hand box but wanted some more info first. thanks josh

guessing a good 2nd hand would cost $400ish

Rebuild 2 to 3-X's that

thanks mate, the guy who has the box says its good but i got no warranty. he said with install less than 750 but how much less i have no idea . anyone know who in vic i could talk to which shops/mechanics/price are good ?etc?