i have found a 74' 455 yy code, with 73' 4x heads 2.11/1.66 valves 114cc chambers, motor looks complete all accesories and brackets minus a/c compressor and breather.
1) is this a good jumping off point for a performance street build?
2)are these heads worth spending the money on to have ported?
3)will the heads need to be milled to increease compression?
4) should the exhaust valve be enlarged?
5) do the 74' blocks have thiner wall, or did that start in 75',along with stronger steel?
thanks for your time and knowledge, tim

I used a '74 455 for my TA clone. I used a Butler rotating assembly and KRE high-port heads. Compression is at 10:1, uses pump gas, and dyno'd at 600 hp. I think it's a good "jumping off".

any 455 is a good starting point if it is a good rebuildable core. You will want to ask if the engine has already been rebuilt. ie. has it been bored already, has the crank been turned already etc. What kind of budget are you looking at for your performance build? do you have a trustworthy machine shop available to do your work. 4X heads are not really the hot ticket for a performance build for a 455. once you start shaving the heads a lot to pick up compression, you will start to have problems with alignment of intake, valley pan etc. in my humple opinion you would be better off finding a good set of rebuildable 6X heads. 4's are getting pretty rare but 8's are still around and pretty cheap. they are 98-101 cc which will give you a better performance compression. If you are new to building pontiacs you should consider getting a copy of Jim Hands, building max-performance Pontiac v8's. And do some reading in it before you start spending money. If you have deep pockets talking to a reputable builder like SD Performance or Butler will get you all you need. A fellow has started a great series of videos on building a pontiac 455 that will likely help you as well. http://www.youtube.com/user/AlchemistLair/videos
I have a bit about my 455 build on my site as well that might help you. link is in my signature.

I have the same block being built right now. I picked up a set of slightly used #96's from Pontiac dude and am putting in a mild cam. So, you have a good jumping off point indeed.

I would say that the #96's heads are the best for a 455 street built without going to aftermarket or round ports.