340 block question

I was looking for a 340 block for my 73 Duster and found a guy who had a std. 69 short block and he also had a 70 bare block that had no main caps. He also sold me in with the deal a complete 70 318 engine with an aluminum 904 attached to it. The crank in the 318 should be a steel crank and if it is, can I use the main caps off of the 318 and send the block to the machine shop and have it line bored. The two engines have the same size crank so will it work. I also need to know if the rods from a 318 and a 340 the same. I have not tore into this 318 yet but I am curious if it will work. Thanks,Bill