I've used both internal boxes and Hawkins flush hunter magazines. If you get your stock inlet for an M5 cut you can go either way. Hawkins makes M5 bottom metal for both styles and Wyatts will have an internal box for the Hawkins bdl style bottom metal.
If you're building it on a long action they are sometimes tricky to get to feed reliably without some tweaking on the internal box. One way around that is using a Wyatts center feed box, they feed like butter out of those. With those boxes the rounds need to be loaded in from the bottom though.
Either way will work, might just take your smith a little time to get them to cycle on an internal box. That's not anything out of the ordinary though as a lot of internal boxes need some tweaking for smooth cycling of all the rounds.