what it a good FAT tire size for my 68 barracuda.
14 rim

I assume you are talking about the rears?

Small bolt pattern or large? Stock rear axles or new large bolt pattern or shortened original large bolt pattern axles?

On the rear 14x8 with 4.5 backspacing and 255/60/14 tire. If it's got original axles, might need 1/8" spacers or run 4.25 backspacing. If rear is all jacked up high, might be able to go higher.

You probably wont find anything bigger then a 245-60 for a 14" rim. They should be a 7" rim. Not sure you will find many 14X7" rims for the 4" bolt pattern.

I am fitting a 275/50/15 currently with 3 5/8 backspace with the stock small bolt pattern offset. It needs the fenders rolled but just barely, if my car didn't sit so low rubbing would not be a problem.

Ages ago I ran a 265/50/14 I believe on a 3 3/4 back space 14x7 slotted mag. I also ran a 26x8.5 slick on a 14x8 rim.

The pic was 1997 or so, I miss those rims. I'm not sure what tire was on it then. Either a 245/60/14 or 265/50/14

The stock wheel well will fit a pretty big tire. I had 325-50-15s on mine with the springs moved but stock wheel wells. There was another member with 295-65-15 or 315-60-15s in the stock wheel wells. Your issue will be finding big tires for 14" rims

Heres the 275-60-15

325-50-15

315-60