To get good performance and economy (for a 440) it would be important to build the engine with quench.

Your iron heads are open chamber so a flattop piston won't get you quench. The KB pistons I suggested are "quench pad" pistons. The raised area on the piston is milled down to provide .040" piston to head clearance. It is slow work because each piston needs to be fitted to each chambe but it has a big benefit - you can run some compression to gain performance and economy without detonation.

With quench, you should be able to run 9:1 towing with a short duration cam like the 260H and build lots of torque. The stock convertor should work fine.

Without quench, 9:1 would probably work with premium fuel but 8.5:1 would be safer.

The other approach is to go ahead and get some Aluminum heads, Edelbrock or 440 Source. With the closed chamber head and a zero deck piston and .038-.040 gasket you will get perfect quench and 9.5 or a bit more compression. If your iron heads need a lot of work, this may be no more expensive.

Buying one of the other engines to build would give you time to rough machine and measure the block so you can get the best possible piston height fit.