We use a 28 bhp that provides 1.2 mbtu/hr on a 30 bbl system.
As with everything the answer you seek is dependent on other variables. Like how efficiently does your HLT, mash tun, and kettle heat up. Our kettle (bottom and side jackets) which is an older Specific Mech heats up faster than our HLT so we heat up water in it while we are mashing in, then transfer it to the HLT. The HLT has an internal calandria that just does not get the job done fast. But you get the point. I think you would be fine with a 39 bhp. I am in the market for a new boiler and the sales folks keep telling me a 50 would be best, but is a lot more expensive. We have managed with the 28 for well over a decade. Newer equipment will be a L of a lot more efficient then the old stuff as well. Good luck.
CBO "Chief Brewing Officer" / Zymurgist
Bluegrass Brewing Co
636 East Main St