It's a long story but I have the guts of a coil spring hi wall action (not a rifle) and a flat spring receiver. The one thing that I'm not sure about is keeping the lever up. Is it feasible to JBweld a block in the hole in the front of the receiver for the flat spring and use it for the coil spring lever plunger?

"He who must not be named" has developed a strategy whereby with close fitting, a flat spring lever stays up without spring tension.  I'm not sure the average gun smith (or below average ones like myself) can expect to be able to do that.  Check with him via the address Whitey just gave you.

Another possibility is to use the flat spring JUST to keep the lever up while using a complete set of coil spring guts to operate the action.  This was done as part of the Italian "redesign" of the high-wall action and seems to work OK, even if it is a little redundant.

The third possibility of course is to swap out the hammer and spring for the proper flat spring ones and be done with it.  I may have the hammer you need and the springs are available new from Frank Zika who advertises in the SSRJ.  You can use a coil spring block with a flat spring, you just have two empty little holes in the front edge of each side of the block.  You may need to modify the bottom of your barrel with a dovetail for the flat mainstpring base, but if you are starting from a receiver only, that point becomes moot.

HTH ~ Froggie