Here's my 1990-ish Univega Via Carisma. MTB components and geometry with 700c wheels. It's become a very fun touring/hauling/cross bike. Kept the original Shimano Exage 500 LX drivetrain (I have ordered a new 7-speed cassette and chain) I have cross tires on it here, but I can tell you that swapping out your knobbies for higher-pressure slicks (currently running Forte Gothams and/or Vittoria Randonneurs 700x32mm @90 PSI) will give you all the road speed you're probably looking for. 48tx12t has been fast enough for me and I'm used to mashing out at 52tx11t on my 20lb old steel road bike with 23mm tires.

I've taken it on a few 50+ mile rides, bare-bones and fully loaded for camping, and I'm convinced that older rigid steel MTB frames with their durable components and relaxed geometry make very comfortable and capable road/touring/cargo bikes. It's great to be able to run errands around town, catch speed on backroads and bike paths, and still brave some single track due to the higher BB. I'm about $250 into this build and almost finished. I've ridden (and ridden with) Surly Cross Check and a Long Haul Trucker, and while they are both great bikes, there's nothing they can do that a conversion like this can't do for a much smaller budget.