This is just thinking out loud for a sense of what's involved and I have not reviewed the FSM -

You have to reset the engine from #1 TDC where it's easier to r&r the vacuum pump to 14* BTDC (#1) so you can lock the IP while replacing the timer.

You don't have to remove the tensioner if you remove the cam sprocket since removing the cam sprocket will give you the slack you need to slip the IP off the chain. You might have to remove the upper guide rail to avoid removing the tensioner. I don't see how removing the tensioner helps since you shouldn't rotate the cam without also rotating the engine. With slack in the chain upstream of the timer, slide a metal or plastic card along the timer cogs to lift the chain off the cogs. Go all around the timer. When the chain is free of the timer, remove and replace the timer. Oops, loosen the left hand thread bolt on the timer before loosening the chain. Don't fit the IP lock until the timer bolt is loose!. Timer clocking should be irrelevant because there are more splines than cogs on the timer <- hopefully not famous last words. Pull out the card and engage the chain in the timer cogs starting with the bottom and working CCW so as to have all slack towards the cam sprocket. If you locked the cam and didn't move the crank from 14* BTDC, you won't have affected IP timing. However, if the splines didn't align just right, you might have to bump the cam or crank a hair to engage the cam sprocket on the cam. You could remove the timer and try a different orientation. I wonder why MB didn't index the timer to the IP.

I'd note cam and IP timing before touching the chain so you have a point of reference. Set the cam alignment marks and read off the crank pulley, then set the IP lock in place and read off the cam pulley.

Sixto
87 300D