Hi
Changing from small to big block is easy.
The frame is exactly the same. All you need is big block front springs , otherwise the body will rest on the tires.:Roll
Finding a correctly dated block is also easy, but bare blocks go for 1500 $ + and probably will need to be bored and decked and .....
Of course, you also need to find the correct heads, intake, carb , alterantor brackets, possibly power steering brackets and , and ....
This is a hell of an undertaking.
Do you have any original paperwork that prooves the big block option ?
If not, it doesn't pay at all to install any originaly dated stuff.
I'm in a similar position with my 68, no paparework proof.
My car has a big block installed, wrong casting number.
I do have the correct heads, but wrong intake/carb setup.
I still don't know what route to go ??
Good luck. Gunther