Welcome to the Forum!
If you're not already aware of it, an excellent resource for y-block Ford information is to Google John Mummert's web site
www.ford-y-block.com John has compiled a boat load of helpful information for others to reference and utilize.
After opening that website, scroll down to the table titled, 'Information Center'. In that table, toggle the heading 'Technical Information'. After that opens up, go down the list of topics and toggle the heading, 'Cylinder Head Chart'.
You'll find that there are ~13 (!) different cylinder heads listed as being used on different FoMoCo y-block applications in 1956! Even on the trucks, ~6 (!) different cylinder head castings were utilized! After finding out which ones your F250 had, you can look at the '58 head listing.
To determine how much difference there will theoretically be in compression ratio, look at the difference in combustion chamber size between the heads that were on your engine and what the '58 cylinder head combustion chamber size is. Knowing the volume of the '58 chambers, you can look at earlier 272 cubic inch head listings in the chart and see what compression ratio Ford ADVERTISED them as yielding for their chamber volume.
Hope this helps!
ADDENDUM: Well, I just looked at John's web site. Unfortunately, the combustion chamber size is only given for one '56 cylinder head series. I then looked at Tim McMaster's web site,
yblockguy.com/ (another good information source). Tim also only has the combustion chamber volume listed for one series of '56 cylinder heads. The information given regarding the one series of '56 272 heads and the '57 272 heads may help you to get a handle on your question. Especially IF the '58 heads you've got a line on are ECZ-Gs...
NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York