Same boat as the op (old Mac classic water ballast). I had a 1990 Honda BF8 (8 hp) and it would barely put out 2 amps at "cruising" rpm.
I now have a 2010 9.8 Nissan (Tohatsu) and it puts out closer to the rated 6 amps over a much larger rpm range - assuming the battery is accepting the charge. It almost seems like this outboard has a regulator as I get near full amps even at low rpm if the battery voltage is say under 13 volts. However, the current seems to back way off as the battery voltage gets up in the mid 13’s range. Not sure exactly how they did this since the motor does not have a regulator – but it works fairly well.
One important thing to also keep in mind.. For that particular boat (26S or 26D), most of the newer outboards in the 8 to 10 hp range will not fully rotate on the transom. This is not always important to everyone but it’s very high on my list of importance as it makes a huge difference for low speed manuevering. I bought an extra long shaft and put a custom transom riser on that raised the outboard by about 2.5 inches and moved it back by about 2 inches. This allowed full rotation plus the outboard prop was still 2.5 inches deeper in the water than the long shaft.
If you go with the single cylinder 6 hp, I don’t believe you have any rotation problems mounted on the stock transom (which I really like). Plus, it’s a fair amount lighter and as someone else pointed out, the transom is not a good place for a lot of weight.
I think the 6 hp would be just fine especially if you’re at lower elevation. I use my 9.8 also at 8600 feet elevation and I can tell I lose a lot of hp but it still has way more than I need. Another benefit to extra hp is that the pitch of the prop doesn’t matter that much. The less margin you have, the more you might need to get every bit of hp out of the engine and then you may need to match the prop better.
