+10 Tire width will get you. 1980s rigid frame MTBs are plentiful, at reasonable to low pricing. And they tend to have more reasonable top tube length. Later MTBs got really long top tubes, making them more of a challenge to convert to drops and get them to fit.
To get cantilever brakes on a road bike, you are talking touring bike, and touring bikes tend to be at the higher end price wise. I've got Fat Franks on one of my conversions, those tires are 2.35 inches wide which is approximately 60mm wide. I haven't seen a road bike that can handle anything close to that.
I have used my Prologue racing bike with 25mm tires on packed gravel trails. Just have to keep your awareness up. Obviously not ideal or anything I would recommend. Had I lost control and wiped out, I would be kicking myself for sure.