converting to 5 lug, problems with rear and driveshaft

[WellBilt Kustoms]

I have a 63 Longbed fleetside, has a 350 with a powerglide behind it. I'm in the process of converting the truck over to a 5 lug, all parts are coming off of a 87 suburban. The rear end is a 10-bolt. The old rear in my truck was a Eaton 8 lug.

I have the new 10 bolt set on the trailing arms, but the problem i'm having is the driveshaft. When I put in the yoke into the tranny, its completely pushed all the way in just to get the rear U-joint to bolt up, and thats even hard to do. This is with the suspension hanging down also. So there's no way at ride height it will work. I'm not sure if the previous owner swapped in a 3/4 or 1 ton driveshaft when he swapped in the Eaton 8 lug rear. The driveshaft itself looks fairly large in diameter.

Can someone measure there driveshaft for me? Or does anyone know what the measurements should be for a stock 1/2 ton driveshaft for a longbed truck?

I'm thinking about either cutting an inch and a half out of mine, welding it back up, then going to get it re-balanced. Or...getting another driveshaft off of something else that will be the right length.