Road Runner Nest

Are 71 and 74 Roadrunner leaf springs the same size in length.I'm wondering if 71 springs will fit under my 74 Roadrunner.

They should be the same length- However I'm not sure it will be a direct bolt in. In 73 mopar started using an "isolated" suspension. This used rubber isolators where the spring clamps to the axle- and ( in some models) used an elogated front spring eye with a oval bushing to further dampen ride harshness.
Now I would think that if you used the 71-2 parts, ( clamps and front spring hangers) that it should work.

That is correct. At least that is the same thing I was told by ESPO when I ordered my new leaf springs for my 73RR.
74RR if you call ESPO, they will be able to tell you exactly what parts you would need to be able to run the 71 Springs. And sell them to you if you want them.

The oval front eye bushings have a tendency to break over time.I would use the round front spring eyes ( 71 type ).

BRIAN