I have some data for you from the oem service manuals. I have a 77 (close enough to a 78) and 66 service manual - here are the weights for rods and pistons.
77 318
rod weight 726 grams or 758 grams
piston weight 594.6 grams
66 273
rod weight 726 grams
piston weight 530 grams (2bbl)
piston weight 569 grams (4bbl)
Somethings to consider
1) The two rod weights for the 77 are for a 318, and a 360. If you have the heavier rod its obviously going to be 758 grams. I am not aware of chrysler having used the lighter rods past about 1972 in 318's, so your rods are probably the heavier version. Weigh them to make sure !
2) 273 pistons used an extremely heavy wrist pin. I believe the reason they did this was to make up the balance weight for the crank. The cranks I think were originally balanced for the early 318 (so called wide block engine), which uses the lighter rod and piston weights as the later 318 (like the 73 and newer motors). Again weigh the piston with the wrist pin to be certain. I think these specs above do not include the wrist pins, rings or bearings.
3) You can press the wrist pins or bush the small ends like the 273 rods were originally. Drilling a few oiling holes in the piston is not a bad idea.
4) Whatever you decide, choose the rods you want to use, rod bearings and piston rings, the pistons and the wrist pins, harmonic balancer, crankshaft, and flywheel (if manual) and have the whole set balanced together at a machine shop. The original balancing job done in the factory was not a truly precise job. You might also consider using 318 wrist pins to cut the piston/pin weight down on a 273. Lighter weight = durablity and quicker rpm. If you use the lighter wrist pins, make sure they are not too long - such that they might protrude from the piston and hit the cylinder walls.
good luck !
