3000 rads isn't much when looking at radiation treatment. I had 70.2 gy ("grays")during my treatment, which comes out to 7020 rads, over twice what you received in 1978. Other people get higher dosages. Plus I've had several CT scans.
However, radiation therapy today is much more precise than in 1978, and radiation oncologists can spare a lot of healthy tissue that they could not avoid 30 years ago.
I'm not sure that there IS a safe lifetime limit when talking about therapeutic radiation. For example, in my case, I know that even with IMRT, my chance of colorectal and bladder cancer has probably increased somewhat. 70.2 grays is not without risk, but it would seem to be worth it in risk/return analysis--a real but small chance of developing another cancer from the radiation vs. a very large chance (50-70%) of curing my prostate cancer.
There has been some recent discussion of safe lifetime limits to diagnostic radiation, especially CT scans. You can find those articles on PubMed.
I'm not a doctor. That said, I would not be overly concerned with issue of lifetime exposure to radiation, especially since the previous dosage was relatively low, so long ago, and likely to a different area of the body. I would be more concerned about the likelihood of success of salvage radiotherapy and the possible side effects. I HIGHLY doubt the radiation oncologist will disqualify you based on previous radiation.
Looking at your medical record, did you have positive or negative surgical margins? What was your Gleason and what's your PSA now? What was your PSA at the time of surgery? Did you have lymph node or seminal vesicle invasion? Those are factors your radiation oncologist will weigh when deciding whether or not you are a good candidate.
Best wishes.