Sleeves in a block initially was a concern of mine, but over time and looking at this issue I've come to realize it's actually a good call.
Why I mentioned a 6,000 rpm range is because of the growing popularity or hydraulic roller cams. They, typically, make their best horse power in the high 5's low 6's maybe for the best price (dollar per horse power). To get the high rpm hydraulic roller you gotta kick up a notch and get the good stuff for a few (or a bunch) dollars more.
While flat tappet's are loosing popularity for a variety of reasons (what, people to lazy to adjust their valves?
) they are a great inexpensive alternative that will take you into mid to 6,000's with no problem. I can hear it now, why would you want to rev that high if you can make the same or more power with a hyd roller at lower rpm? Well, some of us just like a reasonable rpm range, thats all, and to save some bucks is icing on the cake.