It isn't needed, not on engines before '73 or so.. BUT if you're not sure what the function of the tubes(feed from exhaust manifold) and stuff(vacuum lines)are, maybe better to replace it.
Most engines used a solid spacer, maybe with vacuum tap under carb. When EGR was used usually it's spacer replaced the previous type. For a 250 I dunno if said spacer is necessary to set height or angle of carb mounting.
No it needs the adapter. The 250 RBS carb adapter has wider bolt pattern than the 170/200 carb adapter. If you want to do away with the EGR then you will need to plug the exhaust manifold
You asked where I got the plug for the exhaust manifold and I said the hardware store. I got the carb adapter at the junkyard off a 71 Maverick Grabber with a 250 inline six.