Can You Pair Any Transmisson, and Engine Together?

Technically yes, practically not so much.
As long as they conform to the same standard bolt patterns then you’re good.
Understanding SAE Bellhousing and Flywheel Measurements
Eaton 18 speed transmission is available in both SAE 1 and 2 bolt patterns. Most highway semi trucks are SAE 1.

 

Both Detroit and Volvo have proprietary auto shift transmissions. The computers wouldn't interface with other engines. If you were to have a manual, like an RTLO14613, when you need a replacement, you wouldn't need to specify which engine it was going behind.

 

To answer you question .. most likely Yes you can. Do you realize what this idiea might cost you
1st you need to have a transmission with a torque rating high enough to handle the the ft lbs of torque the motor will produce .Your also going to spend some money in replacing things a person tends to over look when doing this in a older truck. Then you also have costs like getting a driveshaft fit... all these little things start to add up.And If you have limited mechanical ability now your goning to have labor involved and a fair amount of it
I've done what your thinking about doing half a dozen times .. and I'am doing another truck now.I also do all the work myself and it is still costly
If you don't mind sharing with us .. what kind of truck ,motor ,& transmission are you thinking about putting together ?

 

Well, yes, but in the past, there were so many factors to consider. Be advised, by saying "older truck", you may as well say, "skirt the regs", and you will for a while, but I bet ALL trucks will have to comply someday. Today, a one size seems to fit all, but back then, lower hp motors needed specific gearing to match the job. Gravel haulers needed much different equipment than road trucks. Older motors are dependable, but lack fuel mileage, if that's an issue. Need more info on what you want to do and where.

 

Practically?

You can order any bell housing for any engine made in the last 25 years. So if you have an engine with a 0 or 00 belhousing and need a 1, then you can find them.

 

If you have a truck vintage were the computer doesn’t control every inch of what it does. Like ‘03 or older. Maybe 2010 or older you can put any non computer controlled trans behind any engine you want. It may not be the best spec to put a 5 speed without a brownie behind an engine instead of a 13 but you can. Like @Catmando said Just got to adjust the driveline to the proper length to bridge the new gap.

 

Be sure you research not only model number but the ratio set which will be a letter usually after the model number. Not all 18, 13, 10 etc are the same. Many just throw a generic blanket and then are disappointed when they discover it’s either too slow or fast for what they are doing.

I realize buying the absolute perfect truck used is an impossibility but buying what you want is cheaper and less heartburn than trying to make something what you want. Not that I have done either

 

Yep
There’s 13 speeds that 13th is a direct drive ,
Some that 13th is a overdrive ,
Some that 13th is a double overdrive etc

it will be alot easier and cheaper to just find the truck that already has the engine trans combo you want . especially if
You’re going to be paying a shop to do it .

the radiator might be different the intercooler might be different etc etc

if you want an older truck , that’s fine , once you get it right it will be reliable , without all the emission garbage and dozens of sensors on it messing up all the time , but it will use more fuel .
Which will be balanced out by the lack of downtime waiting on sensors from
China .

but just decide what rig you want , what engine and trans and cab you like and cast a wide net and find it .