Loss of Top End power

[AndrewM]

Hey all, new to the group and new proud owner of a 71 C20 350 V8. I have been driving without problems for around a week now but just ran into some problems. I filled my car up with regular unleaded (E10 I'm assuming) and when I went to drive it started losing power in 3rd (top gear) when I would get above 35 mph. I limped home but noticed it wouldn't like me to try to get above 35 after that.

I've noticed the oil pressure runs fairly high, and upon inspection, the PCV tube is cracked but still has suction. I replaced the oil and filter yesterday with 10w30. I haven't done anything other than that.

I'm a beginner at all of this so any help is greatly appreciated. Thanks!