Marcos Maniacs

I could really use some help here, 
I picked up my Marcos in the end of August and have been taking baby 
steps with it since I bought it. It is a 3L Volvo powered car 
equipped with a BW35 three speed automatic. I am facing an issue 
with the final drive, I guess. Basically, I am running at around 60-
65 mph @ nearly 6grand on the tack in top gear! I noticed some 
former posts about gearing the rear, is this the answer, if so what 
parts will I need and where should I go for them? PLEASE help me out 
here, my dream car is  is leaving me dumbfounded!

Thanks everyone-
Chris Scafario