Alt. Replacement

If you install a 60A and do NOT have problems, I can assure you that you have "just been lucky." Even back "in the day" I had trouble with my own car, and the larger amp systems had what became known as "taxi/ fleet wiring." This is (dealer) option where the high current wiring was divorced from the troublesome bulkhead connector and ran through the bulkhead on larger gauge wire.

Please read this article from MAD. Even if you do not perform these modifications, this article gives a good insight into the trouble you face

http://www.madelectrical.com/electricaltech/amp-gauges.shtml

The one really really BIG problem in these old girls over all others is the poor connections in the bulkhead/ firewall connector, which were never designed for that much current.