I currently have a slightly built 351M in my 78 F250. I have an opportunity to buy a '74 f100 that has a mild built 1974 390 FE. Which is a better engine to build on and why? I don't think I'll be able to keep both trucks and need to make some decisions. The 74 is a short bed and my birth year and a 4x4 unlike my 78.

 

Sixes
Look at some old Pro Stock Engineering pieces for the FE engines (PSE)
I think the Cleveland design of the Tall deck M motors is superior but the blocks are weak and the 4 inch bore by 4 inch stroke of the 400s make for a low rpm slouch IMO
That said I have owned a few of each and each has its drawbacks
The old side oiler 427s were hard to beat in boats and such, very few made and very rare
However, there are parts galore to make FE power
Might have to have pistons and a cam custom made for a 351-400M
Maybe consider building a long rod 400 with a custom piston
Doing a 427 Chev long rod build now for a 68 Camaro I picked up

 

I like both. In my neck of the woods I have stuck with 335 series because parts in the used parts yards are much more plentiful. like transmissions, intakes, valve covers. Basically everything. I like both engines but I feel that a 335 series shares distributor and bellhousing and other charcteristics to a 385 series, making it much more plentiful in the availability department. 385 series had a much longer production run than others. I feel that power is attainable from both platforms, obviously your 351m is not available to obtain pistons if you are after compression increase, however at what pistons cost now even for a 400 I would strongly consider some machine work like cut the heads, align bore the block, possibly offset grind the crank and depending on your accessibility to a competent machine shop these operations would likely net you a point or 2 of compression each, meaning your 351m would end up in the 9.0:1 range respectively, depending on how far you could take it. A 390 would be a very strong build for a pickup truck with lots of options when considering 360 parts to incorporate into the mix. I would like to have a 390 in my f250 as I like their power curve as well as like the factory 4 barrel option, I just do not see very many available in my area, compared to 460/429/400/351. I build what is cheap and available. My 400 and 460 and 429 collection is very plentiful and I have $600 in the entire collection, and I have dove heads, 4 barrel intakes, 2 barrel intakes, heads, headers, 351m engines, 400 engines, 429 engines, 460 engines, 400 cranks, 351 cranks, and more than I care to mention. Unfortunately, a 390 commands $300+ if it is all there, with the automatic assumption that it needs a rebuild. A 460 or 400 is a $25-$50 take it away. If I can 10 builders for the price of one, it will drastically influence my decision. I think they are all good engines, all have their own characteristics.

 

The 351M/400 engine has much more potential than the FE.