The '60-'64 FoMoCo Fullsize chassis is a good, sturdy platform to build off of, so stick with it if at all possible, unless it is too far gone to salvage.  I have a '62 Mercury Monterey Wagon that came out of a Western Minnesota tree row and was in very tough shape.  It required alot of frame repair to get back into fighting shape.  I ended up going air ride all the way around, 4 link rear and Shockwaves up front.  I also eliminated the Mercury specific Anti-Harsh bushings on the lower control arms and replaced them with standard Galaxie bushings.  The car rides and handles like a dream.  Check out my old build thread on the HAMB if you are interested.  Trust me, it is the ONLY '62 Merc wagon at BTTF's each year.

Do Not put coilovers in place of the shocks.  The shock mounts are not designed to support the added load from the springs, and will quickly fail if asked to do so.  I run QA1 adjustables in rear (stock mount), along with the 4 link and bags, and I have been very happy with them.  I wanted the adjustability since the 4 link setup has quite a bit less internal dampening then the stock leaf springs and I needed more dampening at the shock to control the suspension.

https://www.jalopyjournal.com/forum/threads/1962-mercury-monterey-custom-wagon.959751/

I have been running a Broader AOD in my '62 Merc for the past 8 years with 0 issues.  All road miles, not a drag car by any means, but definitely gets lots of miles banging around town and down the interstate.

The distributor is definitely the biggest issue.  No provision for a thermostat (easily solved) and very small water passages.  I also don't like the large open plenum.  That results in very short intake runners that would likely cause problems, especially lower in the RPM range where a street car spends most of its time. 

Very Cool Kelly. 

I have been watching your videos for some time.  I am very impressed with your pattern and mold making skills. 

If you ever run short of projects, I have a MEL 6-71 Blower intake manifold I need to get cast.  Another one of those unloved Ford engines...

I will glady send CAD and cash your way. 

I hate when this conversation is brought up by Chebbie guys, you all are supposed to know better.  It is an FE dammit, end of story.  Just because Chebbie guys have to reduce everything to their nomenclature doesn't mean we can't be better than that.  How many of us have heard the "Fords are so confusing" or "complicated" whine from a Bowtie fan?  At that point I like to turn the conversation to Buick, Olds, Poncho, GMC, and Cad engines, and all the sudden it isn't so confusing.  351 ci GMC V6 anyone?

Ford introduced 3 V8 gasoline engine families in 1958.  FE, MEL, and Superduty, and the Y-block remained in production through '64.  If we let Bore Spacing and Deck height define block size.

Y-Block: 4.38" Bore Spacing and 9.755" Deck.  Same BS as the later Windsor
FE: 4.63" Bore Spacing and 10.170" Deck.  Same BS as the Lincoln Y-Block
MEL:  4.90" Bore Spacing and 10.482" Deck (at center of bore).  Same BS as the later 385 series.
SuperDuty: 5.25" Bore Spacing and 11.25" Deck. (at center of bore). Similar BS to later 7.3L Diesels.

The Lincoln Y-Blocks had a 10.9375" Deck height, just a bit under the massive Super Duty's but never displaced more than 368 ci.  Packard V8's had a bore spacing of 5.125", again just a hair under the Super Duty's, but never grew larger than 374 ci.  Poncho's have been discussed at length already.  NHRA has made life difficult for anyone wanting to run an engine with a bore spacing larger than GM's 4.84" Are all of those engines to be considered "Bigger" or "Huge" Blocks?  "Big" and "Small" block is simply too narrow of description to be of much use. 

Seems like one of those "You don't know what you can't do if no one ever told you that you can't do it" moments.

I run a 13 lb cap in my Mercury.  I can't speak to whether or not your system will support 18 psi or not.  Remember that extra 5 pounds of pressure is working on large surfaces of many dozens or even hundreds of square inches (expansion tank and rad tanks), and as such will generate many hundreds or even thousands of extra pounds of force.