I don't think anyone has ever converted the lean burn to hemi to be honest. I know the hemi head doesn't seem to have any problems until around the 220bhp mark and then they need to be ported. After porting, the heads are not restrictive at all. If you are having problems with the lean burn head then it could be a worth while approach in having the head milled out to a hemi. It's just finding someone who can accurately do the job. It might be worth while trying the machining on a scrap or second hand head before butchering a perfectly good head incase it all turns to shit.

One tuner here used to be an engine designer by trade and has commented that once the CVH has been uprated with a steel bottom end set up and massively modified valve train allowing it to rev hard, power outputs of over 400bhp and maybe even 500bhp are possible.