Eliminating elephant ears, changing headers

I've got a 71 Duster that I bought as a pig-in-a-poke. It currently has a 400 on elephant ears and Doug's headers. The car has power steering and a v-8 K-frame with stock suspension.

I'm going through the process of figuring out if the engine's any good or not - it has no vacuum - so I may be in the position of replacing it soon. It starts, it idles, and it falls flat on it's face when I rev it. The vacuum jumps up once it gets past the bog, but two different vacuum gauges register zero with the engine running. Yes I replaced the intake gasket and the timing's set at 23* and I've tried it as far advanced as 30* with no improvement. (Any advice on this is welcome)

So it seems like at best I will need to pull the front off the engine to figure out a cam problem, at worst the engine is just worn out and will need to be pulled. The only thing I know about the engine is it's a 400 and has 915 heads. The ears and headers have discouraged me from taking it apart to find out more. Either way it will need to be unbolted from the ears.

I got a wild idea last night that it might be a wise thing to replace the elephant ears with some transdapt mounts and change to Schumacher Tri-Y's or TTIs when I do that. So I wondered if anyone has experience with that combo, and is it as straight-forward as it seems? How much banging will I need to do on the headers to get them to work? Is it likely I can get the headers installed by myself in a no lift garage? Also, every time I hit the key I thank my lucky stars the starter works - can you remove/replace the starter with the Tri-Y or TTI headers?

This is something I would do whether I can fix the current engine or if I replace it. FWIW installing a small block was considered but that will cost over 10k and there's a 10 month backlog at Blueprint Engines, and I've got a good 440 and 727 here that I could just install and be done with it if the mounts and header idea is valid.

Thanks in advance for any info.