I read a series in the 80s and am trying to figure out what the name was. Here is what I can remember about it. Starts out with a guy, think he is a truck driver, giving a lift to an old man. Then a lady is picked up also, but she wasn't really part of the initial plan and is just an innocent bystander. The old man has the truck driver drive him to a ferry that transports them to a leftover portion of Earth where fantasy creatures exist. Since this world was just the leftovers of the Earth they had to write laws to govern how everything interacted. The truck driver was brought by the old man to help save this alternate world. When they make the crossing the truck driver is given a new body that is made from his thoughts, he is described as a barbarian that looks like Geronimo. He has the body of a barbarian, but has to go through rigorous training to be able to use the muscles, since one of the laws is that nothing can be gained without effort. I think the lady became a sorceress. Later in the series his new body dies and he is brought back to life, but another law states that he can't have the body that died or his original body, so he comes back as the body he had when he was in the marines and around 22. He also becomes a True Were later in the series so that he turns into whatever living thing he is close to when the full moon rises. At one point he is suspended over a snake pit filled with snakes with silver nitrate venom. The thinking is that the moon will rise and he will turn into a snake and fall into the pit with the other snakes and be bitten and die because of the silver nitrate.
Joe, the trucker, becomes a barbarian warrior. Marge the lady he picks up becomes a Kauri (kind of the positive equivalent of a succubus), after a one book stint as a virginal sorceress. As you mention, he loses his created body later, due to some body switching, but regains his body from the prime of his life.
The pit you mention is from the third book; it was meant to be his doom, but, due to the Rules the world operates under, there HAD to be a way out. (Ironically, they were no longer in that world, but since all involved were from it, the Rules still apply.)
You have the first three books described in your question, but I must warn you -- the story continues, and, unfortunately, suffers from Author Existence Failure* just as it comes to a MASSIVE cliffhanger, so be warned. (That being said, it was almost a decade between the last novel and his death, so he may never have been going to write the sequel -- but certainly won't now.)
*** Warning; link to TvTropes -- follow it and you may lose hours of your life. That said, if you are willing to follow that first link, here's a link to the tropes these stories embody.