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I have a 54 model with a 235 and a 4 speed saginaw transmission. The driveshaft is stock from a monte carlo (72 I think). I just recently swapped my rearend and leaf springs for some from a 4wd s10 Blazer. The problem is the universal joints are different. The part on my monte carlo driveshaft is a little smaller than what I need for the s10 rear. The caps with the bearings have a smaller diameter. What do I need to do? Do auto parts store carry u-joints that have 2 differents sizes of caps on them? I hate to cut my driveshaft or buy another one.

It is probably a 1330 series and the MC is likely a 1310 series. The cross width is the same if I remember correctly but the journal size is different so it would mean changing the weld yoke on the driveshaft to the larger size. Any driveline shop can cut off a weldyoke and weld on a new one. Done correctly it doesn't require balancing either. Another choice would be to get a stub yoke (the one that slips on the diff shaft)that is the same joint size as the driveshaft. This would be my first choice if it were me. Again, a decent driveline shop will have the application manuals that show what your rear-end has and what your driveshaft is. Yokes are made in many variations so you will likely find what you need.

Yes, there are "adapter" type u-joints to fit applications such as yours. Get into the back pages of just about any universal joint master catalog, and there will be illustrations with dimensions printed on them. Don't rely on the pimply-faced kid punching computer keys at the local big-box auto parts store for a project like that. Find an old graybeard counterman at a real professional parts store!
Jerry

i installed an astrovan shaft on my 55 first series, and was told to ask for a crossover u-joint. i went to a napa store and they new what i needed just by giving them the cap diameters, and the best part was they had the right part at the store and didn't have to order it.

In most metro areas NAPA will have a distribution warehouse with a retail store connected. Usually your odds of getting a needed part there, when you walk in instead of waiting for it to be ordered and delivered days later, is better.