I'd buy a gallon of KROIL, ( www.kanolabs.com ) pour about 3-4 ounces into each spark plug hole, and wait about a week, then put a pry bar to the flywheel ring gear teeth under the bellhousing. In the meantime, drop the oil pan and look for discolored/damaged connecting rods. Losing power at highway speed and refusing to turn over sounds suspiciously like a seized-up rod bearing. If so, that 305 is a boat anchor- - - -you really don't want to get into a crankshaft replacement/regrind on one of those engines!

For a daily driver, buy a Peterbilt 3-axle tractor with a Cummins Big Cam 400 in it- - - -you'll get better fuel mileage!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!