I'm interested in hearing and seeing more.

So, to this country boy the solution is simple, buy a GM powered hot rod cheap and buy a boat. Take the GM engine out and use it for an anchor for the boat. Install whatever engine you want. Problem solved.

Do those early Plymouths have front leaf springs? Do folks typically convert them to a transverse leaf or keep the parallel leaves?

The front suspension swap from the 33 parallel leaf to either the transverse leaf spring front suspension or to any form of independent suspension is a pretty major undertaking. As far as any ride improvement or handling improvement, a modern steering box and a sway bar would accomplish the same improvements, the Mopar parallel leaf spring suspension was a pretty high quality set up if it is still intact. Most of the time, the transverse leaf was only installed because the Ford guys loved them, or the car was going fender less and they didn't like the appearance of the parallel leafs. The independent suspension was mostly a 90s thing when any form of a solid front axle was considered "old fashioned" or obsolete. What most changed to was not an improvement, but was often considered to be a "must have" at the time.

At a wild guess, I would say probably 1/2 of the old Mopars were converted from the parallel leaf to something else, and probably almost 1/2 of those were because the original Mopar stuff was either no longer intact, was in poor condition, or had a problem with the original steering (or lack of power steering).

My 35 Dodge frame came to me with the front axle and springs gone (early 1990s), but the rear of the frame also had a rust issue. I scratch built a frame using the cross torsion bar suspension (it was too wide, I altered the front sheet metal so the tires were under the sheet metal because I lived on a gravel road). We put 77,000 miles on that car in 7 summers, so it worked out OK.