I intend to machine a set of extra high scope bases for a Howa 1500 action. Looks to me like the top of the rear bridge has about a six inch radius. Any body know for sure?

Howa bridge same as Remington 700 bridge. Weaver chart shows that base is for 3.9" diameter, or 1.94" radius. Google tells me that most guys are using a little tighter radius so the base has good contact at two points instead of possibly wobbling around contacting only the area in line with the screws.
Is there a front bridge??

The front bridge is the receiver ring, and easy to measure. The rear ring, up over the trigger area is way more than a 1.94"radius. I made up a template off a 12" diameter lathe faceplate that looks close. guess I'll just go with that.

The "front bridge" is called the ring. The "rear bridge" is called the bridge. Calling them anything else is just confusing. Like having a "hot water heater".
I am in the process of making up some clip slot blocks for Rem 700 scout type rifles in .223. Cant cut the slot into the existing bridge like a 308 action. Too many magazine/feed busts. I just ran some test pieces in delrin at 1.94"R, 1.885"R, and 1.855"R. They all looked fine on the test receiver, but I am going with 1.885".
Remember, the radius is half the diameter.

Thanks for the input Kendog, but I still maintain the bridge on this Howa is a lot flatter than a less than two inch radius.Close as I can tell it's about six inches.

Check Weavers base chart. They both take the same mounts ring and bridge. The 36 base (rear) is cut to a diameter of 3.900"
You said you are working on a 1500. Could it be an old Howa-Dickson Golden Bear? That action is rare as hens teeth and a close copy of a Sako Finnbear. Totally different animal. The top of that bridge is almost flat, which would make sense of the bigger radius you are seeing. All Howa 1500's and Weatherby Vanguards take the same rear base as Rem, which is #36 cut to a 3.9" circle.

There is a formula in Machinery's Handbook that allows you to measure and determine the radius.