Suggestions on performance upgrades for 225

That's a beautiful looking van you have there.
If I was wanting to get that slant six on the interstate
keeping up with traffic. I would replace the 3-speed
with a A833 overdrive. It has an extra .73 or .78 overdriven
gear. In my opinion it is a better choice than going after
higher engine output.
I have had bad luck pushing a slant six at the speeds
people drive on the interstate around here.
Threw the rods on a couple that way.
Better for in town use.