The new centerfeeds are better suited for long OAL repeaters. The have a longer opening by quite a bit.
What Earl did was take an older RUM/H&H length mag box, cut the tabs off, slot the end for the bullets to go through. Then he fixed the mag box into the stock.
Here is a link to his build, some of the pictures are gone but you'll get the jist.
New long Range Savage 7Ultra
If you don't want to convert your old staggerfeed, new Stevens 200's can be had for less than $300. You can probably get $75-$100 of that back selling the barrel and stock.