I drive a 1990 f250 lariat, and it has a 351. Me and my father argue back and forth as to the specific model of the engine, and all the tips to distinguish the windsor, the cleveland, and the modified, have done nothing but confuse us. My father argues that its is a 351m/400 while i believe it is a windsor. Through what ive read, the cleveland and the modified were both discontinued in the early 80's, but have also read several instances where people belived their 90's trucks came stock with a 351m/400. I was wondering if anyone could clear this up.

 

This is real simple.
The Cleveland was NEVER factory installed in any truck and didn't made it out of the '70's.
The 351m was NEVER factory equipped with EFI and didn't make it out of the '80's
Only 1 option left.

 

Thanks very much.... time to rub it into dads face!

 

Also a 351m/400 is a big block. 351w and 351c small blocks. When you combine this info with the info Conanski said, you can determine what type any 351 cid engine is.

 

The fastest way to ID a Ford engine is to count the bolts on the valve cover:
Y block-----2 bolts: 239/256/272/292/312
FE---------5 bolts: 332/352/360/361/390/391/406/410/427/428
Windsor----6 bolts: 221/255/260/289/302/351W
385 series--7 bolts: 429/460
335 series--8 bolts: 351C/351M/400

Here are some good write ups on the different Ford engine families
List of Ford engines - Wikipedia, the free encyclopedia
Ford V8 Engine Identification - FORDification.com

Now you have to be careful when you start tossing around the big block/small block terms. It's easy to label engines from other manufactures but Ford had multiple engine families though out the years and things can get confusing quick. It's better to think of Ford engines by their respective families than as big/small blocks. The Windsor's were very compacted and light weight for their displacement and could be thought of as small blocks. The FE and 385 series were heavy, larger displacement motors and were definitely big blocks. But it can still get blurry from there too. There were 332 and 352 FE motors. That displacement pales in comparison with the 351W/C/M and 400s but they were still physically bigger and heavier and are still considered big blocks. The Y-block weighed in within about 20lbs of the FE yet had displacements of only 239-312 and no one calls them big blocks. The new MOD motors are some of the dimensionally largest motors Ford ever made and in cast iron form pretty heavy compared to their displacement (the V10 weighed more than an iron head 460 and the Triton 5.4 was close to matching it) but I don't think anybody would classify the 4.6 (281 CID!) in my wife's Mustang as a big block. If anything the 351M/400 fits in-between a big block and a small block in terms of physical size and weight and I have even heard them referred to as "mid-blocks" before. To simplify, just call them the M or Modified motors (but don't say 400M or that starts another long argument).

So you can see the terminology can get confusing.

Now as for what you have in your truck, it's a Windsor. Or a small block if you want to call it that. The last year ford put a 351M in a truck was 82. And Ford never put a Cleveland in a pickup. The confusion comes from when they introduced the 351M in the mid 70s Ford marketing advertised it as a Cleveland to capitalize on that engines great reputation (it had just been discounted only lasting from 70-74).Truthfully they are similar, part of the same family, but are different motors. You can read more about them here
Ford 335 engine - Wikipedia, the free encyclopedia
Though the link should start you midway down the page at the 351M I suggest reading that whole page. It gives great insight into the 335 series. They were a great engine, designed during the Total Performance era, but because of political reasons never reached their full potential and went down in peoples memories as a dogged down emission motor.