The oil pressure is good for a stock motor. You've got the same as mine and I've put some long miles on it. 10-15 pounds of vacuum is normal, indicates the top end is in at least fair condition. I'd run a compression check for piece of mind, considering what you have found.
Intake and headers at a fair price, doesn't sound like you've done too bad. The old engine? He said the bottom end was rebuilt. Well that could mean he dropped the pan and replaced the main and rod bearing. Did he have the crank ground?,,your guess is as good as mine. Doesn't sound like he went any further. I'd clean the pushrods, change the oil frequently, and use a good detergent oil.
Larry
68 C20 327 daily driver
63 C20 292 Camper truck
65 C10 ? Custom Cab AC,PB,PS,$flasher
