I am swapping a 1973 360 into my 1969 340 car while I rebuild the 340----water pump is different (opposite side)-can I use the 1969 water pump on that 360--that way I do not have to change radiators

Yes No problem do it.

As long as you use the 69 pulleys and alternator/power steering brackets also as the 69 cast pump and the later aluminum pump use different pulley's and brackets.

Just as Rhino said...timing marks will be a PITA to see but it can be done...

the 69 iron pump is shallower so you'll need a spacer to move the fan forward (no big deal) to where it was with the alum pump. Check bolt length & use spacers/shorter bolts as needed to duplicate what you originally had (good precaution/use Permatex #2). the rad hose/nipple and timing marks will be (both) on the DR side but you can see the marks if you aim the light just right (or find exact TDC & make a pointer for the pass side. Some paint marks on the DR side tab for 0-10-15-17-20 helps with this & some paint marks on the dampener if you use a pointer on the pass side. 0.0632683" around the dampener circumference is (1) degree (cut/measure a strip of white paper)

YOu need to run the timing cover too. There is on bolt hole difference between the old and new style covers. I think the newer block has the hole and the older cover doesn't, been a few years since I did this so double check when fitting up the cover. If that hole is there and is not plugged it will leak coolant, BTDT.

You can get by with just changing the pump which keeps the tcover/block hole orientation as is & functioning but it causes a slight aforementioned prob reading the timing marks. If this was a permanent deal then yes I would want to swap covers & remark the dampener to match and there was a bolt on tab on some 60's SB's (they're available used) that went on the pass side (I have one) & if it bolts onto the late tcover that'd take care of the dampener markings other than a white paint mark for TDC