Changing 1st gear would require more than I think it would be worth. .86 in front of a 3.90 would allow you to go just about as fast as the 235 could break the wind. If the trans is good, I would put it in and run it. If you already have 3.90, you would already have the open drive shaft, so installing it wouldn't be that major of a deal. I would put it in and try it. If you think the engine has enough power to make you go faster, you can always put in a faster OD. I think a .70 OD is the ideal.
