compression leak after rebuild
 

I just had a guy who will remain nameless at this time , rebuild my 1970 2.2L
Heads were refaced, cylinders refaced, supposedly all the standard machine work. It had case savers and new studs installed.
It has one used replacement cylinder. They say it is exactly the same height as the others. All were reconditioned by reputable machine shop.
After 400 miles I started hearing a phat phat sound under load. It is like what an old vw bug sounds like when it pulled a head stud.
It idles and revs fine in neutral. It makes sound only under load. example: 2200 rpm given 3/4 throttle. At 4000 rpm under load it starts to miss also. I have not taken it to higher rpm. I did not even get a chance to break it in.
I towed it back to the shop immediately.
There were no loose plugs or exhaust manifolds. Compression is good all around 140.
Next it will be run on the chassis dyno so it can be placed under load while diagnosing.

What are your suggestions for possible causes and steps to take?
12 month warranty advertised...
Thanks!

Have the head studs been re-torqued? There have been reports of head nuts getting loose. Rare but worth checking.

I have had the experience of using Moly based lube on head studs and with the lower torque
used on smaller displacement engines in combination with stronger studs-which I would not recommend-it's possible for the nuts to back off.

Retorque the headstuds as soon as possible. Don't go on a dyno before that is done.

Edit: Do brake it in properly. Drive it like you stole it from mile 1. Of cause it needs to run alright and clean when doing this.