I converted a mid 90's steel Davidson. You can use existing crank but I changed it for the stiffer BB design. Bought Ultegra, mix of used (shifters) and new (front derailleur) off ebay for about $350. Sold the mint 8-Speed DA shifters and rear derailleur for about half that back. The only thing carry over is the 8spd brakes. Works perfect, pretty minimal investment.

You can use 8 old chain rings, I put a pair of new DA's I'de had sitting around that were 8spd vintage.