A-230 and 8-1/4 rear end question
 

I have a 1977 D100 that has the, I assume since it is a factory 318 truck, close ratio A-230 3-speed. I got it running over the past summer and I've realized the trucks kind of sluggish (a real shocker lol). I'm doing a mild as mild can be build on the engine and was wondering if anyone had any suggestions on regearing. I don't need a family road trip truck so I can tolerate low gearing but I'm not looking for 4.88's like my Toyota pickup. Maybe 3.90s? 4.10s? Any input or suggestions are greatly appreciated. Also if anyone knows of a stamp or marking on the rear end that would tell me the ratio or help me figure the ratio out, I would appreciate that as well. Thanks!

What gear does it have now? I would go with a 3.90 and a taller tire, I have found over the years that a 3.73 gear is a good compromise gear for my streetcars. A 4.10 you’re going to tap out about 60 mph on the highway. 40 years ago I had a 1965 willies pick up, quarter ton, 2 Wheel Dr. with a tornado 6, 3 on the tree and 4.10 gears. Talk about a buzz bomb at 55 mph.

The tag is most likely gone, if you haven't found it already. So, jack it up and spin a wheel while counting driveshaft rotations.

Back when the truck was in the driveway and had an engine in it I looked underneath it and I couldn't find any markings that would tell me what gears it has so when I can clear a space in the driveway I'll jack it up and count the rotations. I took it on the highway once and it screams in third so it might already have lower gears or it was the bad clutch slipping really bad. I do plan on putting taller and slightly wider tires on the rear so I'll probably go with something like 3.73's. I still have to reassemble the engine so I got some time to research and ponder.