Have a look at my rebuild thread which will show you what to expect inside the gearbox (rebuild pics near the bottom of the page).

https://passionford.com/forum/restorations-rebuilds-and-projects/196934-engine-and-gearbox-rebuild-time-update-24-08-a.html

You will proably need a decent puller to remove the 5th gear and apart from that its quite straight forward. If you just want to swap over the case for the series one engine mount then you can leave the shafts and selector mechanism in the lower gearbox casing.

You can get a gasket set from here...

http://www.maintrainltd.com/

One thing to remember is take note of the direction of the slot in the end of the 5th/reverse gear selector rod. When you are rebuilding the box and fitting the 5th gear selector you will have to turn the selector rod clockwise before tightning the pinch bolt. If you dont turn it you will have no 5th or reverse gear or no gears at all as it will jam the selector mechanism.