ARE you sure you are describing 'that old lowly Edelbrock SP2P-4V' Intake you removed ?? (Or am I missing something ?) For 1 a Holley 600 1850 type wouldn't bolt up to that sp2p, so you had to have had some perhaps horribly miss-matched adaptor. Plus do you realise the science behind those "Small Runners" ?? Etc And the Era (Late 70's) & Era of Engines...For the sake of comparison just look at the difference in the size between those intake runners! I’m surprised I was even able to get the old manifold to run a well as it did.
That is a Holly 4160 600 CFM sitting on top. Probably not getting the most out of it (or this manifold) until I make the original 289 into a hot HiPo style motor in a couple of years.
I've inspected & read before & after road tests on the Sp2p's/Holley Z's & in the parimiters they are intended & designed for, they aren't as bad as you are painting - like the quadrajet carb days of old: "...yuck rip off that thing I don't understand", only NOW for many to be seeking out correct #'s Quadrajet carbs like there's no tomorrow !.
I have a Popular HotRodding 2 part road test of them taking a project Smog 350 Chev Blazer & with a 4bbl SP2P inlet swap , with its TORQUE/THROTTLE RESPONSE/VACUUM INDUCING SMALL RUNNERS , headers etc, they eventually gain over 2 seconds on 1/4 mile & add 120hp + 2mpg on a 7.7:1 Comp smogger 350c.i. - for what it was DESIGNED FOR: its NOT a race Manifold, but I think Vic Edelbrock Jnr, when alive, would've been pretty disappointed to read your not very glowing account - that's how products with a lot of R & R $ etc invested get Flamed with the advent of the Internet !.
HOW do we know you didn't have some Massive vacuum leak under that 1850 600cfm - " leading to 15" Vacuum Truck Engine like Running & Idle..."
Always Poetic license in peoples Utube, whoops, www Accounts & Stories.
I think Your junk Ed. Manifold removed would show similar improvements % wise when fitted to a 2bbl 260ci or Power everything 2V auto 302ci with STD CAM OR Edelbrock wouldn't have Cast & sold so many !. ( Think about it )
But seeing you reckon it only pulled 15" of Vacuum etc , I'll relieve you of the lump of scrap alloy off your hands for $10 doorstop value + parcel post cost to OR 97203
Glad your new buy went so well - the operative telling qstn ?: what are you running currently for a Camshaft ? Duration, LCA ? Etc ( never heard of anything going UP in Vacuum from an Sp2p/Weiand 180° Swap out, unless something ELSE was already out to lunch )
Best wishes. & happy hyway cruising.