The best way is with a dial indicator with the heads removed and the piston on the compression stroke. Lacking that, remove all sparkplugs, screw a modified sparkplug (with a 1" rod welded into the bottom of the plug so it hits the piston) into #1 cylinder hole, remove the front upholstery panel and steel cover to expose the front of the engine. Bolt a degree wheel to the crank. Remove the dist. cap & verify that you're approaching #1 plug wire, so you know you're on the compression stroke. Using a wrench on the big crank bolt very slowly turn the engine until the #1 piston touches the modified plug. Mark the degree wheel at that point. Then slowly turn the engine the other direction, until the #1 piston again hits the plug. Mark the second spot. TDC will be exactly half-way between the two spots you found on your degree wheel. I suggest you repeat this on #8 cylinder and use this point to mark "TDC" on the flywheel, since the #8 wire is right next to the bellhousing hole and one can therefore check the engine with a timing light more easily.