I bought my car with the V6 swapped to a 350 already and figured I would identify the motor later on and go from there. I pulled the motor to freshen it up with roller rocker parts, new gaskets and things like that and as luck may have it the engine code on the top back of the block was scratched and grinded down. Now Im stuck because I have no idea what the lift is for the pushrod length. Its a 4 bolt main and Im gonna use the motor as is for a few years while I get the car ready for more power and finish the other stuff so the rotating assembly is perfect. I am gonna include the clues I found and if anyone comes up with anything let me know. My best guess is it came out of an 86 chevy. But if I can be sure then I can start to order my gaskets and other parts.
CLUES
1) M on the block and then missing numbers
2) Number 2 by the rear
3) 627 638 on the back

 

Try and find an old back yard mechanic.

 

Lets see if I can help you a little. Until about 1978 the oil dipstick was on the drivers side of the block then it went to passenger side til about 1988. In 1988 the fuel pump boss was on the engine but not drilled because of fuel injection. 1988 and newer engines use to have 5.0 or 5.7 cast right into them. Now for the casting #s which will positively identify. First and easiest is on the passenger side on the tab directly in front of the cylinder head behind the alternator should be a code there unless the block has been decked. On the back of the engine driver side above the transmission bolts there are a few sets of numbers that decode the engine and one of them sets is the actual cast date of the block. Can do a google search on these numbers or visit a engine shop the like helping. As for you pushrod issue you can either make a tool by cutting a pushrod in half and install a piece of threaded rod and tiny nut to make adjustable or buy the correct tool and measure what size you need. Unless your going to a big cam .480 lift or higher or a lot of machine work has been done to engine the stock pushrods should be pretty close. Good luck

 

If your right about it being a 86 engine it's either a 305 or 350 but the pushrods are the same length If the heads were off it would be real easy to figure out as a 350 has a 4.00 inch piston and a 305 has a 3..736 (3 3/4 ) inch piston which you can check by measuring the bore with a tape measure. And a 638 block might be a 1988 Canadian 4 bolt 305 engine not available in the US. Hers another link http://www.chuckschevytruckpages.com/casting.html

 

Be unusual for a 305 to have 4 bolt mains?

 

That's what I thought also until I did some checking. Seems that the 361979 used in 1977-1979 in Canada HD blocks and another post about the 1988 305 built in Canada with casting 638 .Not sure myself even after doing a Google search.

 

Well the day I bought it and drove it home I did a long nasty burnout in it. Im guessing 350 based off of that because the 305 is a dog. I am gonna be putting it together in the next few weeks with new gaskets, timing chain, lifters, and maybe cam. Im gonna run with the 86 350 thought. If it breaks I go LS. It has vortec heads and a dual plane air gap intake. Gonna upgrade to a 3 core aluminum radiator, and new carb. Im thinking a 625 or 650 cfm will be plenty.