Mark,
Both our 1967 and 1969 Hi-Lo's had/have five bolt wheels. I can't write definitively about the 1967 tire size, but check out Page 16 of the 1969 Owners Manual I put in the HTF Library (at the top of this thread). There is a chart for several Hi-Lo models of that era. I doubt there was any change from 67 to 69, but at least you've got a starting point.
The chart breaks it down into model numbers. As they are now, Hi-Lo Model numbers of that time started out with trailer length. So a Model 209, was the 20' long model in design year 1969. The chart shows a different tire size for each model. It sounds like you're not sure what model you have, so be advised that Hi-Lo measured trailer length from ball socket to rear bumper, unlike other brand lines. (I'm guessing you are new to Hi-Lo trailers, so please forgive if you already knew all this.)
If this is your first trailer, and you therefore have never bought trailer tires before, in my opinion using tires designed for trailers (ST prefix) are a must. Please don't make the mistake of using car tires. Just asking for trouble! (Again, if you are an experienced trailer owner/user, please don't be upset with my assumptions).
Jim