AJ

Did you consider simply using a bigger diameter tire? Thats the cheapest way to go.
I used 700 X 16 tires instead of 600 X 16 thats about 10% bigger in diameter so I drive now 10% faster for the save revs in my 48 Chevy Fleetmaster ( in Holland too) costs about €250 here and you have 2 new tires too.
If you use modern tires you have to use a higher profile as what you have now of a combination of higher profile and wider tire that will increase the diameter too.
Next more logic step is to change your rear axle ratio to a lower one there are kits available in the USA starting from around $450.
Finally changing to a T5 it involves changing to an open system requiring a new drive shaft a new back axle and some modification.That should be the best combination but involves allot of....money.

Jianis