I don't think I'd carry a spare battery, that's just too much weight. If your alternator is hitting 70k+ miles, good preventative maintenence would be to pull your alternator out and have an alternator shop replace the brushes and turn the commutator. Then you don't have to worry about it for another 70k miles. Those brushes are like brake pads, and wear down over time. It's not a matter of if it will fail, but when. Of course, this is speaking from my experience with Paris-Rhone alternators.

It's the original thin copper slip rings (Commutators are on generators.)
that are one of the MANY non-Bosch Porsche alternator problems.
The earlier internal regulator alternators '82 & '83 SCs had inadequate
diode heat sinks which easily fail with heavy loads and bad batteries.
Also, the early 3.2 alternators had bad regulators which would easily
fail with high engine temperatures causing overvoltaging of the battery.
The later (early '90s) replacement regulators are much more reliable.

The slip rings must be replaced and not just turned as do some marginal
rebuilders. At 100K+ miles there isn't much left of the original copper to turn.
Also, the internal regulator should be replaced with a newer OEM (Paris-Rhone)
one and not a junk aftermarket one. The bearings should also be replaced.
The GOOD parts cost alone will be about $75. Therefore, any good rebuilt
alternator (unless you enjoy taking it in & out) will cost over $150 - $200.

Why mickey mouse the job by putting junk rebuilts (Pep Boys specials)
in your car? Pelican should be able to supply most any Porsche alternator,
either rebuilt or new ('82 or later).

Have Fun
Loren
'88 3.2