It will be a lot of work, other than the shifting of all of the lines of code to appropriate position, f90 intrinsic functions will have to be rewritten in f77 syntax, the first that comes to mind is MATMUL(A,B) which is a matrix multiplication routine. It will become a set of nested DO-loops, unless you write a function, then you only have to do it once, but there are others to look for and the compiler will find them for you.
Is the f90 source in freeform format? (i.e. not tabbed out to column 7).
Does the f77 compiler support freeform?
How much code is there? 1k lines, 10k, 100k? or more?