Bicycle Mechanics - Dura Ace RD-7402 OK with 7spd cassette, RX100 freehub and downtube shifters?

Hi, I have a mid-80s Japanese steel frame that I'd like to build up with some older Dura Ace. I'd like to use:

-- downtube shifters (shifter will not be dura ace since I need to find a shifter that mounts via boss on center of downtube).
-- Takagi crank, replace the stock inner 40t with 36t if possible, so I have 36-52
-- RX100 freehub
- -7-spd hyperglide cassette (most likely 13-28).

I'm looking at the dura ace RD-7401 and RD-7402 rear derailleurs. If I end up getting a RD-7402, any issues using RD-7402 with 7spd cassette and shifter?

I'm guessing max 52-28 is too much for the RD-7402 spec, but I would never ride in that cross up.

Suggestions?

Thanks

** edit: if I'm reading sheldon brown correctly, the only way I can use pre-1997 dura ace RD is to mate it with dura ace shifters. so, I'll hunt up some 7-spd dura ace shifters and mount them ahead of the boss

your edit gets it right. 7- and 8-speed Dura-Ace shifters/derailers weren't compatible with the rest of Shimano's line.

Unless you're already committed to old Dura-Ace stuff, I'd recommend that you go with some other Shimano shifters and rear derailers (e.g., I have a Shimano 600 8-speed-era rear derailer and could sell you if you want) because it's easier to find and easier to replace if you need to do so in the future.

52/36 crank with a 13-28 cassette is a pretty wide range of gearing for a double; I'm not sure if that would be too much for a standard road rear derailer to handle. You could get a long-cage rear derailer to be on the safe side here.

I believe all the RD-740x series derailleurs have these specifications:
* Min cog size: 12T
* Max cog size: 26T
* Front chainring difference: 14T

You might be able to go a bit beyond some of these specifications, but the widest gear range that you would typically use these short-cage deraillers for would be 39-53T front and 13-26 rear. Beyond that you should use a mid-cage or long cage derailleur, which would eliminate using Dura Ace deraillers (and shifters) of this vintage.

I've run my RD-7400 on 50x34 with 13-30. That's 33 total wrap. I think the RD-740x is rated for 28 max cog and 29 total wrap capacity so the specs are somewhat conservative and can be exceeded if you are careful about setup (chain length, b-screw, etc.).