I had the same problem with my 235 straight 6. The stock timing mark is on the flywheel and you see it through a hole in the bell-housing. I converted my drive train to a 200R4 tranny so lost the bell housing and the flywheel thus have no timing marks anywhere. I also tossed the stock single groove front crank pulley in favor of a late model two groove one from the 230-250 series straight 6 (direct replacement), but it has no markings on it either.
To get my timing mark back, I first set the #1 piston at exact TDC on the ignition stroke (both valves closed). Then I got a Mr. Gasket timing tab and bent it and drilled mounting holes until I got it to lay properly next to the new crank pulley. They sell a couple variations, this
. You should be able to get one of them to fit even if you need to bend and braze an ear or two on the mounting flange. If none of these can be made to fit, you can simply braze a hunk of small steel rod onto the front timing cover for a zero pointer. Once the tab was firmly in place, held down by two front timing cover bolts, I used a chisel and hammer to cut a groove in the front pulley rim exactly in line with the "0" on the tab. Now I can use my Harbor Freight dial back timing light to accurately set my timing.