You mentioned that the engine was "rebuilt".
13" vacuum is pretty low for a "stock" rebuild using a stock camshaft. Did you use something other than a stock cam during the rebuild?
Another BIG mistake folks make when "rebuilding" engines is not paying attention to quench.
Many oversize aftermarket pistons are shorter than stock ones for pin height placing them deeper in the bores at TDC. This LOWERS the compression ratio when it should actually raise it with the larger CID.
Most head gaskets found in "builder" gasket sets and THICKER than the stock ones which raises the cylinder heads off the engine and LOWERS the compression ratio.
Most engine "builders" either don't know these things or don't address them during the rebuild, so the engines compression ratio is typically lower than it was before the rebuild.
It's also common to replace the camshaft with an aftermarket cam of some sort instead of using a cam with stock specs. If a larger cam (more duration) is used this LOWERS dynamic compression, which hurts idle vacuum, idle quality and throttle response right off idle, etc.
Other mistakes I see with engine builds are not degreeing the camshaft and using "double roller" timing sets in them. Most of the roller sets being sold are JUNK!!! They are "weak" compared to the factory Morse chain set-ups, they do NOT offer constant tooth contact (smoother), not as accurate and the chains can stretch out very quickly.
Adding a high pressure and/or high volume oil pump is another BAD decision I see made with a lot of these engines. Stock oil pumps are more than adequate for all stock and most performance engine builds. Larger pumps rob power and put a LOT more load on the oil pump drive and distributor/cam gear, often wearing them out quickly.
I could add at least a half dozen more items to this list but I'll stop here.
Just throwing a few things out there that may be contributing to your problems........Cliff