This is a common dilemma by today's performance standards...(high rpms at highway speeds with a 3 speed trans).

Consider swapping to a 700R4 (4 speed overdrive) automatic trans. You'll have a couple thousand dollars into it to get there, but you can probably stay with your 3:36 rear.
Or...if a 3:08 ratio rear pumpkin were to be found locally, swapping that in will yield slightly lower RPMs at speed (as CNut indicated) while staying with your Turbo 350.

It's a domino effect to start swapping...just be prepared for that. We've all been thru it and survived.