Slant 6 is no ball of fire, but it really shouldn't have a problem keeping up with traffic or cruising at 70. 2.94 gears work well with a slant, and 94,000 miles isn't that much, either.

I have to wonder about the condition of your timing chain/gears, or if your muffler is plugged (they CAN corrode internally).

Lots of work/expense to swap in a 318; if you're only looking for a stock 318 power level, you can get that in a slant 6 with a 2bbl carb, mild cam, and bigger exhaust pipe.