The first time I had problems was when the guy we purchased from had loaded the car onto the dolly with the rear wheels up. My tires were decent, but weren't 100% and it had just started to sprinkle rain when we pulled out. The first curve I came to at about 45 mph, the rear of the truck tried to come around. I was meeting a Jeep in the curve and I wig-wagged it all the way through, partially on his side of the road. Before I delivered it to Tennessee to have another engine installed, I put the front wheels up. I drove up over the mountains, running at the speed limit without any problems. The pavement was dry and it was a manual transmission.
Many people have pulled them backwards for many miles without problems, but I can feel the difference in how it makes the truck feel on curves. It might be better just to rent a trailer to haul it on. If you rent a tow dolly, don't tell them you're towing a Fiero or they'll refuse to rent to you and if it's an aero nose, (GT, Indy, 86 or 87 SE) take along a couple of bricks and 2 2 x 6's about 3 ft long to put under the ramps to prevent damaging the bumpers when loading/unloading. Tell them you're towing a minivan or a Honda CRV.
3800 swaps have been around for a good while. Loyde Rascoe in Dallas, Tx. was among the first, if not the first to pretty much perfect it. I know very little about the differences of the Series I, II, and III engines, but there are differences in ECM's and transmission control. I recently read that some of the Buick engines have some differences from the other GM 3800's, but what that difference is, I don't know. If you contacted Whodeanie's Customs in Woodstock, he may be able to answer your questions. You can find him in our Members List or call him at 77 Oh, ate fore won, ate one 5 fore.