79 Dodge D150 318 to 76 dodge sportsman rv 360 1ton dully chassis.

My project is at a good start. We'll into.... I was told couple years ago to swap out RV TC if putting in say every day truck. For the life of me I can't remember y... it had a Lower stall or like i believe it higher torque converter it reds at 4k if remember. in the rv external Balance 360s one assume original cus only 50k on it from original owner when it was new. But I scrap metal loads get up in weight the reason of the merge. A week ago took in one at 25k loaded unloaded 6,860 empty truck n trailer. The custom d150 1/2 ton suspension was maxed n the 318 was 85% throttle to keep hwy speed with the wind. I love the 318 I normally take a motor out and put one in because so reliable like the slant 6 got one of them that I attend on putting in a truck at some point and Polly 318 but I got to get all the parts to rebuild it because it's been sitting for years... After that larger load and wind. I got weeks in the 318 so I'm thinking if have to split then I'll swap out or Maybe 360 it tell it goes then 318 but. wind and that's a pretty heavy load I was thinking to stay up for 360 but like 318's gas mileage for like 15 horse difference that's a big difference so what I'm wondering is do I leave the RV torque converter since I will be hauling heavy loads, get a newer one for the loads ill do ,or do I split since it's going to be a daily driver. If i split 318 will be put in. I do not know what the gear ratio is in the rear assuming it's a 3.23. On 16.5 rim 245 tire i think. Hoping for 12 mpgs... assuming 10. What rear ratio would be good for this?

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