does anyone know how much an "x" series 390 nascar block is worth, or does anyone have any information about them?

Most FE blocks made around 1968-70 had an "X" cast into the lifter valley. Not sure what it means, but it does not suggest it is a NASCAR block. There never was a 390 NASCAR block, and the 390 that was used in NASCAR back in 1961 was a solid lifter block with an "HP" cast onto it. I'm thinking you are looking at a common 390 hydraulic lifter block, and that someone may be trying to "puff up" the description to help boost the sales price.

Shoe.

It is pretty rare to find a late 60's block without an X and they definitely were never used in Nascar! LOL hawkrod

Hawkrod

39 Ford Deluxe Coupe
59 Tbird 430
60 Lincoln Premier
(2)62 Tbirds
(3)68 Cougar XR7-G's
69 Cougar 428CJ 4 speed
77 1/2 Ford F250 4X4 w/460 swap
86 SVO mustang
76 F250 Crew Cab
1969 Mach I
look at my cars past and present at superford!

I was going to tell him it was worth at least 10K..... to the right (sucker) person. Besides everyone knows that means "high nickel content".... LOL. (not really).

68 Cougar XR7 street and strip car, 428 4-speed, 3560# of fun, new best 10.43@131.2 1.47 60 ft

that all the later blocks with the X had cylinder walls that were reinforced, More square like in the water jacket area.

saw any diff in a X block than any other. Ive had to many 360 and 390 2 bbl blocks for them to be (special)