Hello and welcome to the forum!
From the Block Casting Number you have posted, 10066086, I have not been able to find this exact casting number.
I have found the following casting numbers that are very close to the number you posted;
10066036, 350 CID, 2 or 4 Bolt Mains, Target Master/Goodwrench crate motor, 2-piece rear seal, "Hecho en Mexico".
No year of manufacture is given for this casting number.
10066038, 350 CID, 1992, 4 bolt mains, Goodwrench crate motor, 2-piece rear seal, "Hecho en Mexico".
Possibly, if you clean the casting number area say with some solvent and a wire brush, it maybe that the original reading was not correct.
Often times these cast numbers are easy to mis-read.
My guess is, the engine casting number is 10066036.
From the Engine Assembly Code you posted, 1M0521 4VP; 1= Chevy, M = Mexico, 0521 = May 21 (assembly day), 4 = 1994, VP Engine Suffix Code = 350 CID, 260 HP.
I hope some of this information is helpful.
Don't be a stranger on the forum, stop in from time to time and keep us updated on what your doing with your Corvette build!
