porting the head can help a lot especially if you are going further than just a bit more than stock power. the key to towing or any high load for a long time situation is a good intercooler, radiator, and oil cooler. as long as you can keep all your temps stable and your brakes are ok it shouldn't care what your towing. if you are interested in a ported head pm me i have rebuilt and ported a few now.

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1985 300D om617: 8mm M pump 175cc 5200rpm, holset he221w @ 30psi, large A2W ic, compounds on the way.
KD9AFT
A&P