I myself would try keep the 6. If you haven't already, try a good super tune, advance the timing as much as you can and add a non-restrictive muffler. I've never had a 235 on the road, but I've had some pretty good luck with 250's and 292's. A lot of people would rather have a newer type 235 over a 250. I have no problems selling 235's whenever I get one.
Also if you need more Hi-way speed, try larger tires on the rear. You've got that great SM-420 Trans to take care of anything the tires might set off on the low end.
By the way, that 235 you have is the same type of engine that was in some road tractors back in the older days. Some of those trucks would haul pretty good loads including new cars to the dealers. .....5 or 6 at a time.
Bill
