Is the core support of a 68 dart the same as a 71 dart

It can be, depending on which size radiator were in both cars when produced
Don't forget about all brands of Mopar's stamping the radiator core support with the VIN sequence # on them

Don't care about radiator location I care about light and bucket location

The head lights buckets are part of the fenders, not the body

Correct.

How can I install head lights on a tilt fiberglass frontend

Im pretty sure Unlimited Products used to sell the fiberglass headlight buckets.
I have also used old steel fenders, drilled out the headlight bucket spot welds, bonded that to a fiberglass fender for headlight and bolting to core support.
I am sure though if you are looking for weight reduction, this is not the best way. I think you can drill the spot welds on just the headlight bucket and bolt or bond that in place.

There may be some variances in rad hole size and maybe some bolt holes, but the core support from 67 to 76 a body should be the same, or at least be made to work

if the fenders are race weight, i don't think you would be adding much to the fender with just the bracketry for the lights.
i think the bulbs, the buckets and retaining hardware is where the weight is.
you could remove a few ounces by drilling holes with a hole saw in the fender bracketry before mounting/glassing those to the fender[s].
then drill a bunch of holes in the bulb bucket.
an ounce here, and an ounce there, and before you know it, you have a pound or two.