Would a MTB frame be too long if fitted with drop handlebars to build an adventure bike? I would like use a MTB frame to build an adventure bike, for the following reasons:


Use disc brakes
use wider tyres
avoid a using a gravel/cyclocross bike/frame because doesn't allow wide tyres and they are expensive


I am worried about the fact that a MTB frame is quite longer than a road bike and a cyclocross bike, and so the top tube will be long and with stem and dropbars will be too stretched and so uncomfortable over a long day on the saddle.

Would these factors make an adventure bike on a MTB frame too long, and so too difficult to ride over long distances? Or are there other considerations?
How long is a piece of string?

Any bicycle is possible, which is the beauty of a bicycle in general. Bicycles can be anything really - roadies, MTB, single-speed, recumbents, even giant A-frame types that have steps to get to the saddle!
Your question is very theoretical and will require a proper re-write because you're not being specific to a particular bicycle or size that we could say ok.. here's what you need.

As an "answer", all I can say now is that you take your frame and your new build to a competent bike fitter and get fitted for the bike! You may find all that geometry might have to be thrown out the window for the sake of your posture, back and skeletal health.

Good luck!