Unless all you have is a 308 case there is no reason you can't use a 307 bearing case. I have built quite a few 307 bearing HEMI boxes and they are perfectly fine. Just because you can't find a 308 bearing at your local parts store doesn't mean they aren't available. I never go to a parts store for a bearing because they are limited to what applications they have in front of them.
I buy the correct 308 bearing from Akron Bearing and they have them in stock. I run a 307 bearing HEMI box in my Savoy and it's working just fine. I would prefer a sealed bearing in my transmissions because the chunks of brass and metal floating around in the gear oil is the reason these bearing fail. My next transmission will probably have sealed bearings in it
Gus