will an eight speed cassette fit on a 6 speed hub and if so would it requier a narrower chin and, therefore, new chain rings?

No, an 8 speed cassette will not fit on a 6 speed freehub. I'm assuming your 6 speed is a cassette/freehub design, not a freewheel. If it's a freewheel, you can mount an 8 speed freewheel on the hub, but you'll have to redish the wheel. In addition, you'll have to check the spread between the drops because 8 speed is customarily 130 mm between the drops and 6 speed is almost always 126.
EDIT: If you do have a 6 speed cassette/freehub, you can replace the 6 speed freehub with an 8/9/10 speed freehub, but you'll still need to redish the wheel and figure out what to do about the space between the drops.

I believe that 6 speed freehubs and cassettes did exist but from what I recall, they are in no way compatible with the newer 7 or 8/9/10 speed freehubs and cassettes. The spline pattern was different and the freehub was likely shorter as well.

Mr. Grumpy, are you sure you have a 6 speed cassette and not a 6 speed freewheel? Regardless, if you must have an 8 speed cassette, the easiest fix is to buy a new 130mm spaced 8/9/10 speed freehub wheel (basically any modern 700c wheel) and either flex the stays out by hand or cold set the frame to make the wider hub fit. Assuming the frame is 126mm now, flexing by hand to 130mm is very easy IME. You will need a more narrow, 8 speed chain but your current chainrings will work just fine with it. The width is taken off the outside, not the inside.

Nope. Not at all. I have a Raleigh Marathon from the eighties and wat to replace the stem shifters with bar-ends and would like to get indexed ones and I can't find indexed bar-end shifters in any thing less than 8 speed.

There's always Ebay: https://cgi.ebay.com/SUNTOUR-BARCON-B...item2a0357c1e1

Man! You are GOOD at the internet!

So does that solve your dilemma?

Mr, Grumpy, OK, now I know what you've got. You have a 6 speed freewheel, not a cassette. You can go with a 8 speed freewheel. I've never worked with one, but I've been told they are available. If you are on the heavy side, you might want to keep in mind that one of the reasons freehubs replaced freewheels is that axles bend more easily with freewheels. An 8 speed freewheel is really pushing the envelope. So if you are up there in weight the freewheel is not a good option. You have 27inch rims on that bike. To convert to a cassette/freehub you'll have to buy a new 27" rear wheel with a 8 speed freehub. You can convert to 700C, but that has its own set of problems with brakes reaching the smaller 700c rim and your front wheel won't match your rear wheel.
Changing an older 27" wheeled bike to index shifting using barcons is not an inexpensive option.
I recommend you look on ebay for six speed Shimano, stem shifters. If you use those, all you'll need is a new Shimano compatible, index compatible freewheel and a new chain. Any bike shop that works on older bikes will have the two latter items. The total cost for making that change should be under $60.

+1 Look at ebay or similar for six speed indexed barcons. I have a set myself on my Miyata touring bike.

Ditch it for a proper 700c bike, IMO.

If you shop around for a while, there are some great bargains to be had. My GF scored an almost unused machine with great wheels and 105 (dual-pivot brakes but downtube shifters) for AU$180.

Something like that makes a much better starting point...

After Roco's list of things to change I'll stick with friction shifting! It ain't gonna kill me. Why in the world would I get rid of a perfectly good bike to get another one that does the same thing? I will alow myself to get a new bike at some point this year but I'm going to wait to see how my biking needs shake out. I'll probably wind up with a 'cross bike for fitness/commuting if I can wrap my mind around a 44 mile round trip! Or I might just keep the Marathon and get a more serious MTB. IDK yet. Any way, friction shifers for the Marathon it is!

Mr, Grumpy,
The Marathon is a nice vintage bike. The frame is 502 Chromoly steel, alloy wheels that probably have bead type clincher rims and decent Suntour components. I had one and flipped it because it was too small. I now have a Raleigh that is almost identical, an 87 "Record". The only difference is your Marathon has a QR rear wheel, the Record's rear wheel is nutted.
I have several very nice sophisticated road bikes, but the friction shifting Raleigh remains my utility bike.