If you are looking for a lot of horsepower, you might try asking this question on the "Tractor Pulling" forum, or search the archieves.
A C-175 engine out of a 350 should bolt into your H. I think the utility 350's ran at a higher rpm (2000?) and HP than the Farmall version.
At over 60 years old, your H has probably been overhauled before, so it may already have over sized pistons. It wouldn't hurt to check the cam lobes and have them reground to specification. You might be able to have the cam re-ground to 350 utility specs and then change your governor spring and re-curve the distributor to get 350 rpms. You would have to throttle back to keep your PTO speed at 540 rpm.