[Skypalace]

I don't have specific knowledge of the sprint engine per se, but the Motorsports recommendations are basically for full-on pro racing, I think they can be relaxed if you're not running as hard.

I went over 120 race hours on my 2004 Cup, and had virtually zero leakdown (3% max), finally had PMNA do a rebuild as I was getting worried about hours. PMNA said they'd never seen as clean a high-hour engine, and to keep doing whatever I was doing. Unfortunately (I guess :-)) the new engine wasn't any stronger than the old one, I was hoping to pick up a little speed :-)

My secrets were: Regular Mobil 0-40w oil changes, warm up the engine before fully revving (I'd warm up in pregrid to 170+ before hitting the track, then do a warmup lap), always do a cooldown lap, and the big one, never miss a shift! Overrevs are of course harder on the engine than anything else. Having full data that's automatically logging all engine behavior is very helpful here.

On the other hand, I've seen plenty of lower hours engines in poor shape.

I'd let leakdowns and analysis of overruns give you the news. If leakdown is good but you're not 100% sure about overruns, it would be a lot cheaper to have the heads pulled and have someone look at the valvetrain etc., than to do a full rebuild.

Re merits of various engines, what is your usage, and what are your goals? Hard to answer without knowing more about what's important and not important to you. Max power? Competitive running in a spec class? Resale? Lowest overall cost? Lowest cost per hour? etc. etc. My personal answer would be to run a stock 3.6 cup motor, as I like running a spec class and a stock Cup car will keep reasonable resale, but that's just me and everyone has their ownd desires and goals etc.