@ NumberSix -- I don't have nearly as many Sheaffers as Parkers, but I'd say that for earlier models have a look at the Sheaffer Balance (I have my husband's grandfather's Oversize, restored and with a replacement cap on it). It's a big pen, but very light for its size.
For the 1950s, definitely look at a Sheaffer Snorkel (but make sure you have one checked out/repaired before trying to fill it -- if there's a leak in the sac you'll end up with the spring rusted (and replacements are not cheap: I had a friend who inherited one, and I said "Oh, I know a guy to do the work the next time I go to a pen show; it will run you around forty bucks or so to get back working...." ; turns out I was wrong because the spring was shot and the repairs were more like $60 -- a THIRD of which was for the spring...).
For an equivalent to a Parker 45? Maybe a No Nonsense pen. Cheap, colorful and low maintenance cartridge pens (I think I may have had one growing up, but it might have been a BP, not a fountain pen).
Ruth Morrisson aka inkstainedruth
"It's very nice, but frankly, when I signed that list for a P-51, what I had in mind was a fountain pen."