421 Serval. Might as well have it ported too while you're at it. research which builder you want to go with and make sure you are ok with current lead times. Builder will set you up with the correct domes for pump gas. Redline, FAST, or driveline clutch kit. Shouldnt need a lockup if you run HD springs but clutch pull will be heavy so lockup is nice if you spend the time to dial it in. Get a pro mod transmission and supporting shift mods. I run a camatv pro mod but again many options, do your research.
28, 33, or 35MM PWK keihins. 28 or 33 will help with fuel mileage, i run 35MM PWKs on my 421 Serval.
Get an adjustable billet timing plate or search on youtube on how to modify the stock one.
Chariot, Matoon, NOS all build high quality cool heads. I run a NOS.
See you talking a lot about having to run stock pipes. Is NH really enforcing those laws? thought their mantra was live free or die lol. Your porting and setup will depend a good amount on what pipes you decide to run. Def talk to your builder and see what they recommend.