Rob; measure, Measure, MEASURE! You will likely need a blank longer than 5". 6" is ideal and gives some wiggle room. Drilling to precise depths is key. I just finished a double closed ended Baron pen. I'll post photos in a day or two with calculations.
Start with a piece of scrap wood (dowel). After drilling, cut the wood lengthwise and measure with your fittings.
One measurement is Key. The nib hole(s) depth.
1. Drill to the exact depth of the brass nib tube.
2 Measure the length and width of the refill spring with calipers. Now pick a drill bit just slightly smaller than the spring. Drill an additional 1/2 the length of the spring. You can make this deeper if needed.