will a 12 bolt 66 rear end fit in a 67 chevelle.

Yes, but depending on whether your 67 has an early or late style rear end, the spring perches may be different.

if the spring perches are different, will that be a big problem?

No. I can't remember which is which, others will chime in I'm sure. The one style has a raised center to the perch that holds the bottom of the spring. The other style is flat and requires a bolt on retainer. I "think" the raised style is later, the flat style is earlier. I believe the spring will interchange. I may have read somewhere (maybe here) that the retainer was still available from GM. Also, seems that the flat style is mounted on the axle a little lower, which can drop the ride height a bit.

With luck, both will be the same for you. if not, all of this can be easily overcome. I figured out all this stuff a few years back without knowing all the details described above before I got into it.

thanks alot.

Older style has to be bolted to a flat spring perch and a teardrop shaped fitting shaped like the bottom of the inside of the spring, approx mid year they changed to the newer style where the bottom of the spring slip over a raised ring to hold the spring from sliding around. I could take pics if you need them?