I have a BW35 from my 1974 164. I converted my 164 over to the M410. Personally, I hate automatics, so the swap was VERY worth it for me. Still, the 164 with a BW35 is a pretty sweet ride.

For simplicity's sake, sticking with the BW35 would obviously be easiest. I'd be willing to sell you the BW35 I've got for CHEAP, but on the condition you understand that it's in marginal condition. By that I mean it shifts, but when it was in my car, it had to be coaxed into shifting by lifting throttle when an upshift was desired. The transmission never slipped on me, so in that respect, I suppose it's at least worthy of bolting in to see how long it lasts. I've also got the torque converter for it.

Regarding conversion to the M410: some people say the M410 for the 164 is easy to come by . . . I respectfully disagree. It took me months of active searching to come across one. Luckily, I was able to pick up a parts car with the M410 in it. Keep in mind, the 164 was the top-of-the-line car in its day, and automatics were all the rage then. Finding a 164 with the M410 is like finding large sums of gold in your backyard.

Here's what you'll need:
Clutch and brake pedal assembly
Clutch cable
Overdrive wiring harness
M410 crossmember (NOT the same as the BW35 crossmember)
entire driveshaft from M410 donor (unless you want to try balancing a driveshaft yourself)
M410 + shifter
Shifter boot
LOADS of patience

For me, the swap went really well . . . but I had an entire parts car to compare things to. Feel free to send me an email at volvoguy164@mac.com should you have further questions about the swap.
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74 164E auto -- looking to convert to manual M410