Ok David, I acquired a 1300 long block yesterday ("F"case).  Knowing that I have a bus and that I would rather increase horsepower than decrease.  What do I need to do to upgrade it to a 1500.  Can I just swap pistons/cylinders?

And what if I wanted to get bold and build a 1776 (Since this will be a spare engine)  Would this case work?

OK, You cannot interchange piston/cyl or the heads. You would have to "open up" the case to fit 1500-1600 jugs.  All the 1300 engine is ,is a stroked 40hp. One year only('66).  It only has 50hp.  
     Don't you have a single-port 1600 (57hp) in Mack now ?   Why would you want to go to all of  the expence and trouble and not gain anything ?  As far as a 1776 goes, you already know what my answer is.
      Oh !  And NO you cannot run "big bore" 40hp P/C's either. different size wrist-pin.
       Do you know whats wrong with the 1300 ?    I know a guy that is looking for some 1300 "stuff"  if you want to sell it ( highly recommended).
       I know that you wanted to hear all of this.

yeah David has it a 66 not a 65 my bad. I't's basically a 40 with the newer rods and crank not to mention it has cam bearings unlike other 40hp.
If ya want a good bus motor think about 88 slip ins or 90's with a 74mm or 76 mm crank. that will get ya a lil stroker that will roll out and unlike the non  stroked 1776 the torque would be great for a bus.

Let me know if you decide to ditch the 40hp idea,I could use some parts.