OK, well going off what little you've said:
If its a BMW V8 then the only boxes which mate are the 'v8' boxes from manual 5 and 7 series v8 cars which come in 5 and 6 speed versions and are rare and expensive. There are various clutch configurations, none of which are ideal and you'll have to buy a suitable flywheel.
If its a BMW v8, there are some off the shelf options for mating the 6 speed boxes (which are identical, but with the bellhousing rotated to suit the engine inclination) such as:
http://pmcmotorsport.com/2,adapter-bmw-m60-m62-s62.html
http://pmcmotorsport.com/76,flywheel-bmw-m60-m62-s62.html
If its another V8 and you simply want to use the BMW boxes because theyre cheap and stong then thats fine, but youll need to sort out a flywheel for your v8 with a matching pressure plate. A friction plate to match the correct diameter for the flywheel and the input shaft splines, and a spigot bearing for the end of the crank to suit the diameter of the nose of the input shaft. Then you need to get the spacing right and make an adaptor plate.
With respect the the BMW boxes, I believe all the inline 6 boxes were the same for years, but changed more recently. Any manual from an E30/36/46 should all swap which is why you see E46 24v engines going into E30s on the original box.