This is a common question and the answer lies in how correct you want your obvious up grade to be (the factory never put a BBC in a '73 Nova so your not going to be able to slide it by the white glove inspection judges at a car show as original).
In 1967 and '68 the factory was fat and happy and operated under the premise that what is good for GM was good for the country. Unfortunately we have lawyers like Ralph Nadir who was trying to get rich by suing the pants off GM for safety issues. (you know those guys on the evening TV who are always asking if you have ever taken or used this product to call us now because we want 2/3rds of the proceeds of a class action lawsuit so we can retire to a third world country). Anyway in 1968 GM was the first company in commercial history to loose a class action court case that resulted in a nation wide recall of a product (in this case it was every Chevrolet V-8 made from 1958 to 1968. The reason the motor mounts failed in service allowing the engine to roll over to the passenger side and jam the throttle open because they used to use a solid steel rod to open the throttle before 1969.
In 1969 GM issued a new design on all motor mounts that will not fit the earlier frame stands. This was to force you to buy new motor mounts of the recently designed interlocking style. (They were not a new design or even a new product. Prior to 1969 all engines over 300 horsepower used different frame stands and motor mounts to keep the vulcanized rubber biscuits used on six cylinders and small base engine V-8's from separating).
Because of this there are four different motor mounts for all Chevy V-8's. Two mounts that fit 1958-'68 cars divided into regular rubber biscuit and interlocking on the high horse engines; and another pair of motor mounts that fit 1969-'72 cars (in 1973 the factory introduced the final solution "clam Shell" motor mount that is still used today on all engines regardless of horse power.
These four motor mounts have different heights and widths and the bolt centerline hole for the grade eight engine mount bolt is different on all four. Because of this many people run into parts mismatch where the motor will not fit (B73nova Moroso: makes three different sized Chevy solid motor mounts (three different part numbers) to fit from 1958 to 1972 and each motor mount has different widths between the tangs, distance from the base to the bolt hole and the tang length is different on each one to match the frame stands the factory used over the years).
The factory is all about interchangeability but there are different parts for different applications (based upon GVW among others factors such as horsepower, even the speed rating of the original equipment tire affects the parts you have on the car: whether you are aware of it or not). Motor mounts are one of these black holes that information falls into and never gets out of (are you paying attention Dr Hawking?)
To answer your question (if I put this up front would you have read this monolog?) the 1969 kit is what you want. The 1967-'68 kit uses the very rare odd-ball size motor mount that is not sold at any corner auto parts store like the 1969 version is.