There is no gear set available for your current rear that would do the trick.
The T-5 (S-10 five speed) would be a stretch in a truck that weighs 5,000 and can carry another 10,000. I would not expect the transmission to last very long.
There is a transmission known as the MY-5 or something similar to that, someone will come along with the right info, that is a 4-speed with an overdrive 4th. Its hard to find one that fits a GM bell housing, but they are out there and might be a good option. It was used in GM and Dodge pickups late 80's early 90's. I may not have the details right, but again someone who knows will help out I'm sure.
There are the newer pickup 5-speeds which all come with some issues, but may be what you need.
You can also look at changing the rear axle. If you find a post by Grigg and check his signature line you'll find he has adapted a later 10 lug rear to his 2-ton. He could help out with details such as whether or not it supports the original wheels or not, I'm pretty sure he is not using the stock wheels.
And another option to consider is using an axle like a Dana 80 and cutting off the ends and welding on the ends from your stock axle. Provided you measure carefully and get the hubs in the right spot you can then redrill the hubs to take the Dana axles.
That gives you a modern rear with more ratio options but keeps your original brakes and wheels.
Other options are to find a Brownie, which is an axillary transmission providing overdrive, a Gear Vendors overdrive, or a Ranger overdrive.
And since you are running a 250, the GM overdrive automatics are an option too.
This may not be an exhaustive list, but it should give you an idea of the range of options.
