Ok, finally got some answers back from the Old Fart experts at The510Realm who use M445 Cams regularly. Yes you can use Flat Top pistons with no valve reliefs. Got that from any engine builder who regularly built 510 motors with L24 FT pistons. I would check the piston to valve clearance though, just to be 100% sure.
He also mentioned that new stock valve springs could be used, That I'm a little wary of. You do have to check for coil bind very carefully with stock springs. I think I would follow Rebello's recommendation on valve springs. You don't need anything super strong, but you do need a slight increase in pressure if you want to pull 7,000 RPM or so. IMHO that is. When I was Racing 510's and 240Z's back in the day, we always upgraded to the Roadster springs. The Roadster springs would handle more lift before coil bind and were a bit stiffer than the stock L series springs. They are still available at DatsunParts.com
Standard Roadster springs:
Uprated aftermarket Roadster springs:
Of course you may want to pressure check the springs that are on that head. may have been upgraded already. That would be nice
.180" Lash pads are the size you need. Stock Steel retainers are fine. You may want to trim the bottom of the retainers for more retainer to stem clearance. Check your retainers first. PO may have already done that. Or you can use a shorter seal. Ford V6 are popular as are Datsun A12 seals.