I have a 1974 spider that has 4 disk breaks. The breaks are very spongy, I have tied bleeding them with no luck. Any suggestion?

you probably have air trapped in the compensator

How long has it been since the brakes were last serviced and the rubber parts replaced?

Note that the rear brakes can't really be bled with the rear axle unsupported as there is a compensator valve that adjusts pressure to the rear brake calipers based on how much weight is on the rear axle so your jack and stands need to be under the rear axle and not the body frame in order to bleed the rear brakes.

If the brakes have not been bled and refilled faithfully every two years then there is a good possibility that the rubber brake lines have been compromised and are in need of replacement. Over time they can become plugged or the liner can shred making it impossible to bleed the brakes and get a firm pedal.