You have been sold a sock of poop as far is info. Unless that frame has had some extensive work done to it, as well as the rad support and inner fenders on the truck you have a LOT (read that
LOT) of work. We are talking adding a room onto your house kinda work.
First off, unless it's an S-10 frame it is WAY too big.
There is no such thing as a frame "swap", it requires a LOT of fabrication to pull off making a frame fit another vehicle, it is the rage apparently in 2000's to do this, I don't get it, I have seen way too many of these that look like every loving crap.
There are zillions of kits and parts to upgrade your original frame. Awesome springs that make it ride beautiful, dropped axles, bolt on disc brakes, there are ZILLIONS of kits out there. You can swap to independent with a bolt on kit in one weekend!
Look at what just one place has.
Classic Truck Chassis & Suspension Parts - Speedway Motors, America's Oldest Speed Shop
You want this thing done by May, sand blast and paint (or simply have it powder coated they will blast it for you) your original frame, get that Speedway bolt in crossmember and your done. If you want to shop around for a Mustang II crossmember there are tons of them, get some good references (personally I have no idea if that Speedway one is good or not) but there are TONS of options for these trucks. Changing frames changes
EVERYTHING. Running board mounts, bumper mounts, the rad support has to be cut all up, the inner fenders, the steering has to be changed, the motor has to be moved from the stock location on the frame, there is NO SUCH THING as a frame "swap". It is no different (not a tiny bit) than trying to put a fender from an 84 pickup on your truck! "COULD" it work, sure with enough work. But making one from scratch would be a lot easier.
I have owned one of these trucks for 40 years, drove it every day, drove it all over Cal, it WORKED with the stock suspension and that was with crap old junk. Rebuilt with the new super ride springs and such, they drive sweet.
REALLY think hard before you start such a HUGE project as modifying the wrong frame (or fender) to fit your truck.
And as far as money, by the time you rebuild the suspension and find that you need another rear end anyway and the steering box is shot and on and on you save NOTHING. Add up REALISTICALLY what it will cost and you will find that buying some nice rod parts that were made to update your truck are no more money than that frame "swap". The "swap" is simply a money swap, time swap and work swap. You swap money time and work spent onto the wrong frame to force it to work, that's all.
Brian