....and found out I can't replace them w/o the heads coming off....
i think that´s not true. I put on new heads on my engine, torqued them down....and forgot to put in the roller lifters (LA Roller Block, but afaik the same lifters than the magnums). I was able to put the lifters in with the heads installed. It was close, but not a problem. Just for your information.
Michael