Typical Axle to Crown height for an 80mm fork is about 450mm. Feel free to correct me if I'm wrong but if I recall correctly an old rigid mountain fork was about 390mm(?). Every 20mm takes a degree off of the head tube angle so your old school 70.5º bomber suddenly ends up with a 67.5º head angle and it raises the bottom bracket and lengthens the wheelbase by around 18mm each as well. I've never done it and it's probably still rideable but it isn't anything I would do.

Finding a used 63mm fork and pairing it with a steeper 71º head angle will keep things at a more reasonable 69º and 12mm. I have a old bomber hardtail that currently runs 69º and 8mm higher than stock and it's kinda fun to ride around guys on XC full suspension. It doesn't do sharp switchbacks well at all but they can't believe I can ride downhill type stuff on an 80mm hardtail. It's actually the bike's geometry that gets it done, not the travel.