What % decrease in value non matching numbers...the Jag engine

I'm about to sell my 1955 XK140 SE OTS which I've owned since 1971 in Southern California.

When I bought the car engine block was trashed. The local Jag dealer, a friend, gave me a very low mileahge 3.8 out of a Series 1 E that had been wrecked years before. A 9:1 vs 8:1 3.4 original but non repairable original block. Engine runs superbly with original (recently fully rebuilt) C head.

What is the "negative" value I should anticipate from purists citing non matching engine block?

Will other upgrades such as LED tailghts, Pertronix distibutor, Gustafsson starter negatively impact sales value? I am willing to discount by a/m factor if purity is key.
Thanks