These shops are proficient in Buick engines:
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TA Performance Products Inc.
The 455 Buick cylinder heads are limited by pushrod location and porting them can prove frustrating. Most will drill out and sleeve pushrod holes. The center head bolt boss should be drilled out and a substantial steel sleeve installed to protect port integrity after porting. Larger valves can be installed as dictated by desired power levels. I provide this information as a reference to the amount of research needed for a good engine build.
I do not usually make specific part recommendations over the internet. The above listed shops can assist you in component selection for your unique build.
By now you can see the importance of planning your engine build.