6-cylinder engines are inherently balanced, which goes a long way toward getting good high-RPM performance out of them. A good weight-matching job on all the components and high-strength rod bolts will allow you to run considerably higher revs than the engine was originally designed to run. Other high-rev tricks might include lightweight pistons, and a corresponding reduction in crankshaft counterweight size. You might also investigate the possibility of offset-grinding the rod journals for extra stroke length, combined with a connecting rod and rod bearing for a smaller journal size. the hot-setup 350's are using a Honda rod bearing, and a custom-designed rod for this type of situation. With a little trick cylinder head work, you should have a real screamer. Remember this old adage: "Going fast costs money. How fa$t do you want to go?"
Jerry
