1) Firstly, your 318, if it has even compression, has the torque to pull that caravan trailer. I know because my Dad's '84 did for thousands of miles even with the 2.45s, at least on the prairies around here..
2) but if it's a lean-burn, you gotta prove that ignition timing advance is still working.
3) and if you keep the LB carb, that the heated intake air system is still working
4) that the heat-crossover under the carb is still open and functioning correctly
5) that the Exhaust system is flowing decently.
6) that the cooling system is still able to handle a load.
7) and that the fuel delivery system is up to the task.
8) These are ALL things that should be done anyway, to help prevent the likelihood of you getting stranded, out on the road.
If I was in your situation;
1a) the first thing I would do, is determine what your rpm currently is/will be, at your planned cruising speed. By the math with 2.45 gears, and with 205/75-15 tires, this should be about 65=2000 rpm at Zero-Slip, which means in Lock-Up. (But with 2.76 gears it is expected to be, 2240rpm)
1b) so this will lead to proving the rear gear-ratio. and
1c) to if the engine can pull the Caravan, at the expected cruise rpm.
1d) if it cannot tow the trailer at the selected speed, try faster/slower. 200rpm can be 7>10hp, which at 65mph is a very significant number.
If it still cannot maintain speed, or if the trans waffles back and forth into/out of, lockup, then,
Now we come to a fork in the road;
assuming the ignition timing is correct for whatever rpm the engine is at; yur gunna need more power. Your choices are; more airflow or more rpm.
In this case, more airflow means super/turbo charging. so, barring that, you would need more rpm.
For more rpm, your choices are;
a different bigger-number rear gear,
a higher stall convertor, or
a different transmission, or just a way of manually unlocking the Lock-Up feature. which might get you 100 to 150 more rpm.
What I would do; because I have the parts, is;
install a different trans without a lock up. that should give me an easy 100/150rpm higher cruise. But if that still isn't enought, I would install a higher stall convertor. Something engineered for 2400 to 2600 in your application.
The higher stall will allow the engine to rev to a higher rpm, where it makes a bit more power. This will make getting underway easier and quicker, and allow the engine some freedom in finding cruise-power, and possibly even help in Second gear passing.
FYI, I have a 318 powered A904 with a 2800 convertor, and I absolutely love it with any rear end I have tried, allbe it, Not with anything less than 2.76s. and to be fair, I run it with a 4bbl and dual-exhaust.
the Next things I would probably do, is;
2) defeat the lean burn and install a custom calibrated old fashioned distributor, with a high-powered coil. and
3) a small-port, high-velocity intake, with a tiny 4bbl carb on it.
I suppose, when you get right down to it, in this application, maybe swapping in a better suited rear gear might have been cheaper, but that would require at least 3.23s to match the 2600 stall, and it would NOT have the same multi-purpose function as the higher stall, and as you know, 3.23s would cruise at 32 % higher rpm, so 2630, over 2000 calculated,with 2.45s, at Zero slip. But one good thing about 3.23s is that 55mph in Second gear at WOT, is gunna be about 3400, so you would have some good passing power, whereas with the hi-stall and 2.45s, the math comes to maybe 2600. Even the 4bbl would not be helping down there.
And finally, a 360 will pull that load easy enough at 2000, and the long stroke360 running at 2000 should, with a little tuning, go further on a tank of gas, than a 318LB running at 2000 at near WOT. Yes, I confess, I have one of those as well.
I would try that 360 with all the 318 top end on it, including the cam, which, by itself should increase your Cylinder Pressure from around 135@sealevel to ~150 ; which is a very significant plus 11%.
Ok, I convinced myself, 360 it is, lol.
As for you,
Those are some tuff choices.
I suppose, a junkyard rear end is likely the cheapest choice.
