i'd used the electronic ignition. it's fairly straight forward to modify the wiring for it. however, i'd suggest recurving the distributor for better/optimized performance-- it'll have a smog tune on it.
it'll be similar in performance, the compression ratios aren't that far apart. however, check that the heads don't have smog ports. if so you'll need to plug those.
reuse your flex plate and all that-- it'll bolt directly to the 318
the later 318 has a different front cover with the timing marks on the driver's side to correspond with a balancer that is marked differently and a waterpump that exits on the passenger side (instead of the driver's side in 69). naturally the radiator lower hose is placed on the passenger side.
so you'll need to take into consideration how you want to tackle this-- probably the easiest is to re use your front cover and such and just remark the 318 balancer.
but if you need a new radiator, this would be an perfect opportunity to upgrade everything...