sorry got a little over kill on the info.....
we have a 76 HD F350 delivery truck that we converted from 460 with C6 3sp auto
it did about 3,000rpms at 60 and never got more then 7 mpg ever......
i had one trip of all in town driving and it was in the GAL PER MILE RANGE
to a 300/6 and 5sp
are empty weight of the truck went down a lot doing this swap the frunt end of the truck is now a good inch higher then before. its likly about 500lbs lighter..as that dino auto box and big block are SUPER HEAVY!!!
and top gear is about 2,000 @ 60 drop to 4th when loaded and you still have the power to move a load.
its a work truck so we stayed with the 4.10s seinc its a 14,000 GVW truck and needs them when loaded, but we found a verson of the ZF s5 42 trans with a deep 1st gear thats the small bell housing to fit the I-6 engine. it was sold from about 1987 to 1995 or so.
if i did it over i would find the HD verson of the trans with the external slave versus the internal clutch slave the one the light duty has. other then that there the same trans..
best i know in the usa, 1987 was the first 5sp year there where a few light duty 4sp O\D tranys from around 81 to 86 with real tall 1st gears but other then those there all drect drive 4speeds 86 and older.
in the US there was two versons of the light duty 5sp around 88 to 95 one has a silly tall 1st gear and the other is a granny like your 4sp. the tall geared ones are commen in 1/2 tons and the granny 5speeds are normally only found in light duty 3/4 tons. bolth tranys have a 25% O\D so the choice is how tall of a 1st gear do you want.
what im getting at with all this is, this is a one ton, and is about 7,000lbs empty and can get 11/13 mpg. loaded to the 14K limit, its about 7/8 mpg your ride should be blowing the doors off those numbers...
there is a baby bro to the 300/6 and its a 240/6 for your light 1/2 ton that mite be a good combo...as your loaded weight will likly not exceed my empty number unless your pulling a trailer...that is...
the 300 and 240 are truck engines and are considered big blocks in the word of I-6s in cars there was a 170, 200, and 250 engines..there the small blocks..
1 86 T\D trooper with rare GEN 3 rods TRANS FIXED
1 86 4WD 5sp pup is 2.3L gas, but plan on 2.2L diesel repower
1 91 trop, long term plan is a group buy of imported Isuzu 4JB1-T 2.8L I-4 engines, hoping to get price down to 2K not 3K plus
1993 sidekick my MPG toy, epa rating 26.
i get 29/31 with stock drive train.