Here is what I did, thought I'd share as I just helped a guy with a $5000 built engine that had this problem

Problem:
Manifold is letting air in at the rear, maybe front of engine. The problem is a very common on 302 / 5.0L engines that are used hard. This can cause all kinds of weird problems with the engine. The gaskets Ford uses on the front and rear manifold are not ideal, they will suck in. Just give them time. Trust me and check you engine at these locations.


The Fix:
Remove intake manifold(s), remove the manifold gaskets (Take your time and remove it well). Replace the manifold gaskets with Ford parts but just use the gaskets attached to the heads (sides). Do Not use the gaskets for the front and rear (They will be a cheap upside down gray rubber "U") trash them. For the front and the rear you NEED to use the Ford RTV. Do not use the stuff you can buy at NAPA or anywhere else. The Ford RTV comes in a small "can" that will fit in a cocking gun. On the front of the engine lay two 1/4" or more thick beads from head to head. On the rear lay four 1/4" beads from side. Yes, it will be very hard to lay this much RVT, around the PVC valve you might have to go with less. Just lay it on thick so it will squirt out. Let the RVT stand and form a skin, I think Ford says 1 hour, if it doesn't let it stand 1 hour. After an hour lay the manifold down from front to rear as you drop it at a 45-degree angle. Torque the bolts down and let the RTV harden, as recommended by Ford. Other words don't start the engine for at least 24 hours, maybe even 48! This should do it. Just make sure you use only Ford parts and RTV!
This is what a Ford Asset ASE Mustang Guru did on my engine.
Yes, there are other way's of doing this but this is the best I've seen.

While I was gearing up to do my engine swap, I pulled the intake while the motor was sitting in the garage. Although the engine only had 47,000 original miles, it was leaking quite a bit of oil from the back of the intake. I took extra time and cleaned the hell out of everything. I did use the gasket, but took extra care to make sure it was centered on the "rail" of the block. I use Hi-temp RTV gold on everything I do. It also was able to setup for a few months!

So far so good, but if it fails I'll give your method a try. Thanks for sharing.

Every aftermarket intake will tell you to do the same thing. You can even do it without removing the intake. Just yank the old gasket out, spay with a can of brake cleaner, fill with RTV. Yes you can use any RTV.

Also note there is no vac in there. It is only to seal the oil galley for the cam/lifters. If it fixed your friends vac leak, it was when you changed the intake gaskets. Also to note you should run a bead of RTV around the water pasages from the heads to the intake.

Sounds good but dude, "cocking gun"? Sounds like something I definately do not want to use. Caulking gun is what you want :D