I agree that you want a current balun. I've made several 4:1 current baluns for OCF antennas using 43, 61, and 73 materials.
The crossed (Trask) winding style produces higher common-mode (CM) impedance at the upper end of the balun’s frequency range while producing lower CM impedance at the lower end of the range. You should use the style that produces greater CM impedance at whichever end of the range is giving you the most trouble. I always have trouble at one end or the other even with 80-10m designs.
Type 61 is probably the best choice to try if the desire is for a broadband balun. On paper type 61 material is good to 200 MHz, and it can handle power at 6 meters without overheating, so you're in good shape to start experimenting. As Steve, G3TXQ pointed out, the problem with 61's relatively low permeability (v/v type 43) is that it takes many turns or tall stacks of cores to achieve adequate CM impedance. Tall stacks yield a better solution than many turns because they produce lower interwinding capacitance. Type 61 is a better choice than type 43 because type 43 gets too hot running against an unbalanced load with an impedance mismatch. A lot of the commercial designs use type 43 because it reduces the amount of ferrite required.
I use binocular designs formed of paired core stacks to keep more of the winding inside the cores. This raises the common-mode-impedance to shunt-capacitance ratio, which is a good thing at the higher end of the balun’s frequency range. The extra surface area helps if there's heating as well.
I usually put a 1:1 common-mode choke on the transmitter side of the balun to increase CM impedance further. This is an ugly solution, but it works.
There are some photographs and comments on materials and construction here:http://www.flickr.com/photos/flint-hill/sets/72157633429796001/
I would suggest dividing your stock of FT140-61 toroids into two stacks depending on how many you have. I have had to use as many as 8 toroids per stack to get decent choking impedance on 80 meters while using few enough turns to work at 10 meters. I don’t work 6 meters, so I have no direct experience there. Perhaps start with 4 per stack.
The best way to measure loss is to wire two of the 4:1 baluns back to back, take voltage measurements at the input and output and calculate the loss from the squared voltage ratio.
You can check for heating empirically by hooking the balun up at the transmitter and running a high-duty-cycle mode (SSTV in your case). Check the balun temperature with your thumb when you’re receiving. You'd like little or no heating even with extended operation.
There's really no substitute for testing the common-mode impedance once you've built the thing. This is tough to do with a 4:1 balun unless you have an analyzer that allows you to null the text fixture before testing the balun. It’s possible to make qualitative comparisons with a non-nullable analyzer like my MFJ-269.
I've never used them, but local six-meter people speak well of Hy-Power's GU4-HF baluns. They're rated for matched loads, so you'd want to derate them for real-world usage. There's little enough ferrite even in their 5kW balun that I'd probably get that one rather than one of the lower-power versions. I believe you can build a better balun yourself without too much trouble.