So, I'm swapping an 8 1/4 rear into my scamp, replacing the
original 7 1/4...

I have recently procured a set of leaf springs from a '74 225 duster that have SIX leafs! (The ones under the car now are a four leaf configuration.)

Out of curiosity, I was on the JC Whitney site looking at new springs; and found that they offer a direct four leaf replacement.

So the question is:

Slap them in with new bushings.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

That depends. If the '74 Duster springs are sagged, there's little point installing them regardless of bushings. If they're in good shape, then base your bushing decision on the condition of the '74 springs' present bushings. 4-leaf vs. 5-leaf vs. 6-leaf really depends on how you'll be using the car and what you want/don't want in terms of ride and handling. I don't like the 4-leaf springs; too billowy and jiggly.

JC Witless sell springs from John R Spring Service on Dequindre drive near Troy, Michigan. Perfectly good product. But much better customer service from Espo.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

The duster seemed to have proper ride highth(although, is there a proper A-Body highth?), but I was not able to road test the car.
The springs themselves look pretty good (to an untrained eye), other than a little hint of red under a solid base of brown dirt.

The car will be driven 'spiritedly' with relative frequency.
Not a daily, but darn near.