OK, if the builder sent it, then I suppose they know what they are doing. The Joe Gibb oil is good, along with Brad Penn which are both formulated for the "older" engines. Harbor Freight gauge could indeed be a little off.
The cold weather will make the oil thicker and give a higher reading as will a new engine until it gets a little wear on it.
The hi-performance Pontiac engines use a 60 PSI pump (the family car got a 40 psi pump). I had an issue with oil pressure and used 20-50 plus a little STP just to keep it alive knowing I was going to have to pull it for a rebuild. Got some high oil pressures when winding it out to near 6,000 RPM's, the high oil pressure seemed to have a tendency to push oil past the oil ring seal on the spin on filter. Slowly over 7 years and 20,000 plus miles not even the STP helped and one day I fired it up and the gauge read "0". Gave me the excuse to side line the car and do a complete frame off resto-mod which it is still undergoing.
The ethanol laced gas should have no impact on it. Never heard of any problems during a break-in.
Don't know if they recommended a "break-in" procedure or if the engine has already been broken in by them? You want to do some acceleration (pressure) as well as deceleration (vacuum) runs with it to seat the piston rings.