Export custom formatted expressions from Mathematica - formatting

Question

How can I get Mathematica to export/save/write a text file with proper Fortan77 formatting, that is, 72 columns and a continuation marker on the sixth column?
I am using Mathematica to generate large and complex analytic expressions, which I then need to insert into pre-existing Fortran77 code. I have everything working correctly in the front end of Mathematica with FortranForm[] and
SetOptions[$Output, PageWidth -> 72]
However, I can't figure out how to get Mathematica to output correctly to a text file. I want something like this:
MM11 = mH1**2 + (g2**2*v1**2)/2. -
- (g2**2*(v1**2/2. -
- ((v2*Cos(phi2) - (0,1)*v2*Sin(phi2))*
- (v2*Cos(phi2) + (0,1)*v2*Sin(phi2)))/2.))/2.
...
but get either this:
MM11 = FortranForm[mH1^2 + (g2^2*v1^2)/2 - ...
or this:
MM11 = mH1**2 + (g2**2*v1**2)/2. - (g2**2*
(v1**2/2. - ((v2*Cos(phi2) - (0,1)*v2*Sin(phi2))*
...

Answer 1

This is a job for the surprisingly little-known Splice function. First, you make a template file, with the extension ".mf", like so:
file = "test.mf";
out = OpenWrite[file];
WriteString[out, "MH1 = <* form *>"];
Close[out];
Now when you use Splice, Mathematica will automatically replace everything between the <* and *> delimiters with its evaluated form. So if you set
form = 4 + b9^2 + c1^5 + c4^5 + h10^4 + j2 + k10^4 + p10^4 + q5^5 +
q8 + s3^3 + s7^2 + t6^3 + u3^2 + u9^3 + x8^4 + z2^3;
and call
Splice["test.mf", PageWidth -> 72];
which will automatically infer you want FortranForm output from the file extension, and which allows you to set PageWidth as an option, you will get a pretty decent result in the automatically generated file "test.f" (note the new extension):
MH1 = 4 + b9**2 + c1**5 + c4**5 + h10**4 + j2 + k10**4 + p10**4 +
- q5**5 + q8 + s3**3 + s7**2 + t6**3 + u3**2 + u9**3 + x8**4 +
- z2**3
Look at the docs for Splice for more options (changing the name of the output file and the like).