chevy bb vacuum issue

I have a bb402 that I just put some aluminum edelbrock performer heads on to upgrade my iron ones. After installing the heads I noticed that while driving above about 25% throttle I would hear a sort of whistling sound out the exhaust, but this was only under heavier throttle than cruising. Another new issue to the new heads was that I had trouble shutting down the engine. It would always kind of sputter a few revolutions before actually stopping like it was dieseling or something. I do have a vacuum guage in the cab that I monitor regularly, and at idle I would have 15in, and if I downshifted while driving to use engine braking to pull the rpm's up it would go up over 20in.
Well, this weekend I was out and all of a sudden I noticed a major idle problem where it felt like it was going to stall every time I came to a stop, it never actually did stall but just barely. My vacuum at idle was barely 10in, and if I got up to speed and downshifted to pull up the rpm it would only get up to 15in. So basically I lost 25-35% of my vacuum all at once. I was hoping a vacuum line came loose but unfortunately that was not the case.
I'm thinking maybe I made a mistake when adjusting the valves, which is why I had strange whistling and shutting down issues, and that maybe this weekend something happened to a valve or lifter that made the problem worse and my valves are not closing properly?
I have a hydraulic flat tappet and I followed standard procedures when adjusting them.
Also, this weekend the shutting down problems were much worse, it would sputter for a solid 5 seconds at least and then finish off with a big puff up through the carb.

did you do a mid lift check to see if the push rods are the right length for the new heads sounds like a intake leak ?

what exactly is a mid lift check? I do have a dial indicator, and I'm about to pull covers and start checking those types of things. But I guess I don't know what pushrod length would have to do with lift? Wouldn't it be the same lift regardless, the rockers would just contact the valves off center if the rods were the wrong length?
Should I just put a dial on top of the pushrod, or place it on a rocker?