Im from Durham its near wallingford. I go to chuck and eddies all the time its pretty close to me. There is a few of those trucks there. There is a lot more GMT 400's but I was there a couple weeks ago pulling parts and there was about 3-4 box bodies there. I know a guy that I get parts from that has tons of them. he sells them for $50 bucks each (the double cardan type) if you want his number let me know. hes out my way. But if you dont have the bolt on flange and you already had the shaft changed to 1410 and thats what you are going to bolt directly to the tcase, then no those shafts wont work unless you go to a 1410 yoke at the Tcase too. The 1410 dwarfs the 1330 1350's that these trucks had to beging with so there is no way you are going to be able to bolt this thing up now. Why did you go to 1410 at the rear joint? And are you sure you have a 241C? The front joint is the one that is going to be the first to snap. The front rear u joint will almost never break if you use the double cardan type. I would look to get a flange on that tcase or just swap it out for a case with a flange and use the double cardan shafts that are a dime a dozen. you can find an np241c for about $75 on craigslist all the time. Actually the guy that has the drive shafts had one last time i was there. I get 1350 slip yokes from Tom Woods Drive shafts that go right on the 32 spline shafts and I can just swap the slip yokes off. I use Tom Woods Super Flex joints to get more clearance and they are 1350 and very strong. I have never broken one.
the box body shaft, with a 1350 32 spline TW slip yoke and superflex joints are your best combo for that truck with that tcase hands down.