Hi Hive, As a matter of fact I have a set from a Galant I want to install on my truck. I grabbed the roller rocker assembly, hydraulic lifters with the cam and nuts and bolts, valve cover and cam gear. I was getting tired of the flat tappet noise that mine makes and was going to replace to the gallant parts. These parts will all fit, but they won't interchange. By this I mean the cam the galant is obviously different with the thinner lobes and if I am not mistaken the cam gear also needs to ride along with all the parts because of the cam gear pin.
What I have found with the G63B, is everything can be mixed between the 2 Shortblocks, everything except the block itself and back plate on wideblock & narrowblock. The intake manifolds will interchange to upgrade. The heads are also basically the same except for the married parts that are installed at factory. The subtle differences like the narrow cam lobes verses the wide, short cam pin verses long, changes in cam gear depth thicknesses, etc. make the differences. My motor I have in my 1st gen 5 speed 86 Mits MM narrowblock is in fact half 2nd gen G63B with many internal engine parts from an automatic transmission 87 wideblock truck.
But, what your doing or want to do is exactly what I have planned to do. The flat tappet will give more horsepower from what I have read, but they can be a little noisy if not adjusted correctly. I have to send the cam to them which will only be a labor charge.
On my flat tappet cam grind, Schneider charge $125 for the grind which was not bad, but to get the best out of it, you need to get distributor re-curved on advance because the stock advancing is no benefit. Among other mods like headers or weber will help also as well. Weber 38 would be beneficial if you drive freeway a lot or like to push the pedal down. But, you can also get a carb sync kit to remove the progressive 32/36 carb linkage setting and a special mod jetting kit customized exactly for your truck to increase HP even more.