Sounds like you dropped an exhaust valve.
Lot's of bad talk here about SBT but I just ordered my 12th engine and I never had to send one back. I had to call and get a gasket set sent, that was suppose to come with the motor. But I've had nothing but good luck.
I stopped rebuilding motors some years ago because it's so cheap just replacing them. I still piddle around with my HO 350 cu.in. as a hobby.
But, by the time you finish all the machine shop work and replace everything, you'll be way over what SBT charges for another motor. I"ve heard that their pistons were not of high quality but even that I have in question. Only because with a company that has bought out 2 other companies in the U.S. in the last 4 years, their reputation is pretty high for their products. Also, if there was a problem with their 4-TEC OEM remanafactured engines, I don't think it would be cost effictive to rebuild with a set of crappy pistons.
The 12 engines I have ordered have all been either 587, 717 or the 787 models. Nine of them were 787's. So, I have no experience working with them on the 4-TEC's..............But for cost, I'd check into it.
I'd also be careful about just doing an upper end repair. If you dropped a valve while on throttle, you might have metal pieces through the engine that if not found, could cause another failure. So, it you elect to do the upper end only, I'd at least take the engine completly down to do a complete inspection..............:cheers: