With a 42 objective I dont think you are going to have a problem with getting too close to the hammer if the front clears the barrel as it must.
Id prop that scope up in there with some cardboard and tape and measure how tall the rail to scope body needs to be with the bell clearing the barrel by about 1/16" or a tad more (not too much) then measure some rings in store.
FWIW, my H&R rail is the thickest one I have, so that gains you a little if yours is the same. Regardless, with a large bell scope you are going to have a pretty heads-up position behind it.