You might consider a compromise.

Get a larger 40 size trainer, and power it with a 60 size motor. Some of the 40 size trainers are actually closer to 60 size bird.

For Example: Sig Kadet LT40, 70" wingspan, Great Planes PT-60, 71" wingspan.

Putting a 40 sized motor in some of these larger "40 size" trainers will limit their performance, and as a result, limit the time you'll be flying them.

Of course, if you can't fit it in your vehicle, to transport it to the field, then you have problems....
Easily solved... get a larger vehicle.