Unless you expect to run more than 300 bhp,there is no need for a counterweighted crank or for that matter special rods.
You wont have to rev it more than 6.5k if the turbo is sized correctly,I personally wouldn't use NOS for the fact of it not being available all the time.
But I would use thermo coating on the pistons and given a choice an aftermarket engine managment,so you can control the spark and fuel better.
Heads do restrict the performance as do carbs, I seem to recall we couldn't get more than 200bhp with ported but unwelded heads(we got some from Eurorace which were welded and ported to take it up to 265bhp)
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