>"If it's a standard road triple, the BCD for the middle and outer rings is 130mm, and the smallest you can go in that size is 38 teeth. Below that, the bolts would interfere with the chain."
A standard road triple back in the '80s had 110/74 BCD, as another poster noted. With a 74BCD you can go down to a 24-tooth chainring.
If looking to upgrade the rear, I'd highly suggest moving up to at least a 7-speed cassette hub from your 6-speed freewheel. Cassette freehubs tend to be stronger due to the way the axle is supported, a factor when under touring loads. Plus, it's easier to remove a cassette even when it's been subjected to a lot of torque, like it would be under when climbing with a load. You could either pick up a used wheel that takes 7-speed cassettes, which is likely to be cheap since they aren't in much demand, or get a wheel with a hub that will take a 7-8-9-10 speed cassette to allow yourself upgrade options down the road. You can run a 7-speed cassette on an 8-9-10 speed hub by using a spacer.
If upgrading the wheel, you'll likely also need to change the spacing in your rear dropouts. You probably have 126mm spacing back there, and modern hubs either use 130mm (standard road spacing) or 135mm (MTB spacing, also often used in touring/hybrid wheels). If you have a steel frame, it's no a big deal to move from 126mm to 130mm. You can either have this done professionally by a bike shop or you can just spread the dropouts by hand each time you put the wheel on/off.