I always wonder when folks mix up leftover parts from different generations... it's really asking for it.
The vacuum can was shot, and probably the timing is was not advancing fully. It is very easy to get another vacuum leak when swapping manifolds, there is no way to know how much the heads have been milled, what condition the new manny is in, etc. Sealing requires a lot of careful inspection and preparation.
On an old Pontiac manifold -- and this came up in another thread down the board in the big car section -- if the choke tube that goes through the crossover has a leak (46 years, ya think?) I can almost promise you that you have a West Virgina EGR system at work.
Plus, unless the heads have been off the 455, you don't know for sure the head gaskets are sealing fully, or if the valve job is any good. You need to check compression, do the leakdown and check the cooling system for traces of combustion gas. Are the carbs calibrated for a 455? If you pull a tune up from a 389 and think it will feed an extra 66 cubes... not so much.
Everyone who can fog a mirror knows now that you have to check impeller clearance religiously and get it perfect or you're trying to mix margaritas in a swimming pool. And PS is it the right impeller? What is the pulley ratio? And do you know for sure you have bled ALL the air from the system and there are no steam pockets still sizzling in there.
Tracking all that down takes time, it ain't no fun, but it costs a lot less than new radiators.
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