i have a 5.0 engine that i bought used that has gtp40 heads. the previous owner had a 3 angle valve job done one the heads plus some other stuff, which i dont recall.. my plan is to install this engine in my 64 falcon which has a 74 model 302w. what i was wondering about is there any difference in as to how wide the frame rails and aprons  are as compared to a 65/66 mustang?
one other than one this other engine i am wanting to put in my falcon, i don't know what year it is, and the only number i find is behind the intake manifold which i can't make out due to the poor stamping, but in the lifter galley in the center are three X. are there any other places to look to find any numbers at all ?
 

Those Falcons came with 289, which is identical dimensionally to your 5.0.  Are you concerned the engine won’t fit in the car?
Pretty sure Bullet Bob helped put one in a Falcon for a friend a few years ago.

The fact your block has the xxx cast into it tells you it is a roller 5.0 from mid 80’s or newer.

The codes you need are above the starter. There will be an engineering number containing 6015 and a date code cast in raised letters newr where the bellhousing bolts on.

not concerned with pulling what's in the car and installing the other one. I guess what i'm concerned with the most is there a header that I can use, that any one else has used in a 64 falcon with gt40p heads that will clear everything. I'm fairly sure the headers that are on the car won't just bolt on with out some modification. I have fabed up stuff on the car but my welding skills just are not up to the challenge but I do have friends that said they'd do the welding. at 73 it seems to take me considerably longer to get things done,