OK, here are some pix of four of the several jugs I have that I've set on this GT5 engine lower I have.
The piston in these pix is a "Fred Piston" it has the nice machined ramps at the transfer port openings, when the stock unmodified piston is used the ports aren't as open as you will see in these pix, the only cyl/jug that still allows the ports to be nice and open is the cyl that has the larger rectangular transfers, the others are the standard GT5 cyl, the BGF 40mm wide intake jug and the PK80/40mm stroke type cyl which is commonly found on ebay, it's what I've gotten in the past from BGF when I ordered a standard jug with 32mm intake bolt pattern. these have fairly good ports that clean up nice but have smaller transfers than the two rectangular port jugs I have and the transfers in the BGF jug are lower and dont open as much @ BDC on the GT5 engine.
NOTE: The GT5 cylinder is the only one that doesn't need to have the deck shaved/milled for best compression and best squish area for a after-market head like the "Fred Head" or some of the others we have available.
One thing I will add here is that Fred told me a while back that if I ever need a cylinder/jug deck milled down, all I needed to do was mail it to him with $20 and that would cover shaving the deck to correct height and return shipping, I'm not sure if this is a service he does for anyone needing it but it may be so if someone needs the work done so they can use a pk80 type jug on their GT5 engine I'd say contact Fred and ask him and he might be willing to do that for whomever may need it done since he can do it quick and easy in his shop.
Here is the link to contact Fred: https://www.shop.crmachine.com/contactus.sc
Now for the pix