Whoa! Slow down there big fella!
Let's look at RCA 28 a little more carefully. As Jim pointed out the shoulders are very narrow on the tang. Yous are quite deep and do not appear symetric. I don't see and guidelines on your tang. Heed Mike's advice - draw it before you cut! The arched finial on the tang in the picture does NOT have any portion with parallel sides. The tang in the picture looks much shorter, overall, than yours. (The tang maker expects you to cut off what you don't want.)
Also think of where the tang screw is going to go. Picture 28i shows it. Is the tang you have going to put the tang screw right through the trigger? Your tang has to be short enough to clear the trigger. But the location of the trigger is governed by the location of the lock and that, in turn, is governed by where the vent is. Have you drawn this all out on the stock so you know exactly where everything is going to go?
RCA 28 is a very interesting riflegun! It is certainly a worthy goal. Think thrice, measure twice, cut once.
Best Regards,
John Cholin