The only other transfer I have heard about that compares to the 205 is the Atlas II. I have no other knowledge of this but know they're expensive. The SM420's came in earlier Chevys, like 1966 and earlier. The SM465 started in 1967 (or 1968?) and replaced the SM420. It's a lot easier to get parts for and I have heard a little stronger, but I have no first hand knowledge of this.
Yes, the NP205 is heavier than the D300, but not by a huge amount. The real weight comes from the tranny, which is quite heavy. I can lift a separated 205 (not easily), but not an SM465, even with no oil in it. I'm no weightlifter, 5'10" and 210 lbs, but I'm no couch potato either. BTW, I lifted the whole SR-4/D300 assembly from the Jeep without any help as a comparison. The adapter mating the two is also heavy, everything being cast iron. I would not ditch the D300 for the NP205 unless you're swapping the tranny as well. And then it'd only be to save $$$. If you need another D300 I'd look for one, it'd be an easier swap, unless you're looking for that low on the tranny, your profile doesn't mention what tranny you're running.
It's a good setup, the easiest and most bulletproof I could find at the cheapest expense. It has the granny low Dave mentioned and won't wear out (the gears in it are HUGE!). I did a lot of research (like I always do) before deciding on this setup, and came across guys putting over 750 hp to them without any problems. I had to do something as my SR-4 chewed 3rd and the others were marginal at best (hold in gear and very little synchros left).
The main drawback is the lack of low gears on the NP205, which is only 2:1, not the 2.62:1 of the D300. It was this reason (and the length of the Scrambler) that I'm deciding to go with the Doubler, which mates the NP203 (strong chain driven 2:1) to the NP205 (gear driven 2:1). All I have to do is find an SM465/NP203 setup and I'm in business (1974 Chevys). But this may have to wait until the SBC 350. I'm in the process of searching for a 203 setup now. Maybe I'll do both next winter.
I noticed you're running a 360, you'd need a new bellhousing to mate the two (I think, correct me if I'm wrong). BTW, the SM465/NP205 setup was 3.5" longer than the original SR-4/D300 combo. You can see how much larger it is in the pic. I added a 2" body lift to get it to clear (1" would do) and had to cut out the center well for the transfer case shifters.
Do yourself a favor before buying it. Sand down and clean up the adapter between the tranny and transfer and look for cracks. The adapter is where the combo is supported and having the weight on both sides tends to crack the case, but it's usually just a hairline fracture, and only over the top. New adapters are $220, but I found a used one in CT for $95 (with spare shifter, hence twinstick!). All the ones I found in junkyards up here were cracked or broken, which is probably why they were there.
'81 CJ-8 Scrambled!
GM151/SM465/NP205 twinstick/7" Lift/33" TSL's/REP 8000/RS9000's/IHC Scout II D44's 4.10's F&R W/Lockrights