I just converted a Centurion LeMans and am using the stock crankset.
Unfortunately the aluminum spider is a one piece unit with both a 110mm and 185mm BCD and an integrated chain guard. Not to nice looking.
I am using the inner chainring, a 40t that bolts to the 110mm pattern (dashed line in diagram below)
I am considering cutting the outer portion of the spider away - mainly for looks. Here is a diagram.
I'd cut it at the red lines and of course finish the cut ends so they look nice and are smooth.
Would this weaken the spider? The outermost ring shown is the 'integrated' chainguard, maybe it provides some rigidity? But a chain ring bolted on the spider does as well.