TH200-4R is a bolt with the crossmember in the TH400 position and a TH350 driveshaft. The 3.8 shares motor mounts with a Buick 350 though that engine was not offered in 65. I don't know if you can bolt them into the positions for the original 300 or 340 but there is a good chance. A guy has done an LC2 into a 65 as documented by Car Craft and there have been many others. This is a pretty bolt in deal and if you're keeping the carb its even easier as you don't need to mess with the EFI tank, pump, wiring, or return line. The 3.8 isn't exactly a potent motor in non-turbo form but if you really get your act together you can make decent power out of it but nothing on the order of tweaked LC2 power.
If you are thinking the later Series II 3800... well thats a totally different story and I don't know where to begin other than you want an L36 out of an 96-03 F-body with corresponding tranny, either the T5 manual or 4L60E auto, You'd need to relocate the crossmember with a new set of holes for the tranny and a shortened driveshaft and I have no idea on the motor mounts.
I'd also worry about your stock 8.2 BOP 10 bolt, its better than a chebby 8.2 but still not up to 8.5 10 bolt standards. Best bet would be to get a Turbo Regal 8.5 and either swap over lower control arm mounts and get some hybrid uppers, I think South Side Machine makes some, for durability.
Best of luck but I think you'd be better off doing a 455 swap for more power and TA and Poston make the stuff to make it a bolt in swap, keep the TH200-4R though, just you'll have to build it to match the kind of torque a 455 can dishout.
Just my $.02,
