How's your transmission mount? Having too little fluid in transmission could cause it that when you shift from drive to reverse, it won't engage and you won't have reverse. Don't think too little fluid would cause clunking. Too much fluid would overflow out of your dipstick tube when it got hot. You'd see that as it'd drop on your headers and smoke much from the hood.
Gets hot through the "metal" hurst shifter? So hot you can't touch it or it's uncomfortable?
-------------------------------------------------------
351W-powered 1979 Ford Mustang notchback
Stock 5.8L under 4" cowl, C4 w/ shift kit
Holley 750 cfm, Edelbrock Performer RPM intake
1 5/8" MAC shorty headers, Al driveshaft
2.5" Off road H-pipe, 2-chamber Flowmasters
Front: 225/60/15, Rear: 255/60/15 Eagle GT II
Weld wheels (15x6;15x8), 8.8" Rear w/ 3.55s
14 x 4” K&N air filter (getting the Xtreme setup someday)
"Red, thou art my companion. Hasten now your quickened metamorphosis to Green that I may conquer all who dare abide there beside me. May they be left thither behind burnt black." ---Fox Body