Dumb transmission question

So I’m really looking for some help here because I haven’t got a clear answer yet. I own a 1989 Peterbilt 379 with a Caterpillar 3406B paired with a Eaton 10 speed trans. I need to swap out the 10 speed for an 18, but there are so many different codes and variations when I’m looking at the remanufactured 18’s online, i’m scared I’ll buy one and it won’t fit on the truck. Does anybody know which eaton 18 speed I can pair with this motor?

Thanks in advance

 

Thank you all so much! I’m not a mechanic by any means and these shops down here in Florida definitely take you for a ride if you don’t know what you’re talking about lol. To answer @MLC Adventures ,it has 3.70 rears at the moment but I may change that depending on how the 18 does. I like to be able to cruise in the sweet spot at around 85 and just split up the top two gears

 

Man you’ve taught me so much already and just these two threads. After seeing this I’m definitely going to go with the RTLO. And that’s good to hear higher torque trans are more common in the market. The truck is getting an inframe right now so I figured I’d go all out and spec it how I’d like. The pump is already turned up, it’s getting a performance turbo and bigger nozzles so something durable is a necessity here. It also looks like I’m going to have to re-gear it, that shouldn’t be much of a problem though. I really appreciate you

 

smaller gears are definitely in the question at this point but I’ve never ran tall rubber, would probably be a good idea though for the extra ground clearance. As for the speed, I run out west alot and take very good precautions, if you know if I mean lol