I'd be leery. Sure, it's only 3 miles, but your flat out for 3 miles. Additionally, keeping the head cool would help, but everything is happening in the block - cam, tappets, valves, piston, rings. Plus, you're exiting your exhaust through this same chunk of iron - the block is, in effect, an exhaust manifold.
I've always loved flatheads, but I've never built one, so maybe I'm speaking out of turn. But it sounds problematic.
2 cents - worth maybe half.