Can't help specifically since I have a '77 with the 454 engine. I believe the older alternators were about 65 amp. I suspect this is under powered for the loads, especially when the house batteries need charging as well.
The problem I ran into is that there are two sizes of alternators. Large or small case. Mine has the small case. Apparently mine could only be custom built up to about 92 amps. This does seem to help. If you have the large case you are probably able to go up considerably, unless your are limited by the vehicle's capacity to carry the additional amps. To go up in case size requires different brackets.
The motorhome is stored at the general aviation airport at Anchorage, Alaska. We use it for summer relief from the Texas heat after having been Alaska residents for many years. When I stored it in September'09, the alternator output was erratic. My objective was to determine if any of the parts stores in Anchorage has a replacement in stock. I will work the problem when we arrive there 30June.
It doesn't sound like you have to worry about the house batteries with those long days. I doubt you'll have any problem. Both sizes should be available at most all parts stores. I replaced two before my present and paid well under $100. They inexpensive enough that I carry a spare with us.
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