As far as tips for using the ford chassis - I should probably suggest to use the chevy (Impala/Caprice) as they are closer dimensionally. The crown vic worked fine, but needed 4 3/8" added to the frame - the town car may not need this. Basically, I went to the bone yard and flipped a crown vic donor upside down and cut out a piece of frame and used that to get the extra length with diamond bracing and gussets.
The Vic wheel track is also slightly wider, but makes no difference using backspaced wheels. (the wheels on my car in the pic are Velocity R178 18" w/ 40 backspace, no issues - even with the air ride lowered down. The crown vic fuel tank is mounted almost vertical - the town car should be the same, will (does) protrude up into the trunk, but is unitized and easy to mount. I shrunk some angle into horshoe shapes and built the trunk floor up and over the tank. The stock gas cap is now too low - I remedied by mounting a Jaguar gas cap above the driver's rear quarter panel.
I used everything from the vic - so I narrowed the dashboard at the glovebox and retained all of the gauges, climate control, etc. The blower for the heat and AC must be relocated (inside passenger's front fender). The air intake for the 4.6 must also be relocated, I removed the silencer and ducted into the inside of the driver's front fender with a K & N.
The body mounts used are ones I trimmed off of the vic I cut the piece of frame from. The 4.6 has a terrible lower radiator hose location, managed to find a 2001 Expedition in th bone yard that had the right bend and went to Autozone and got a new one, trimmed to fit an Ebay bought aluminum radiator.
The key to disecting the vic and making it run again when rebodied was the FORD Electrical and Vacuum Shop Service Manual (purchased on ebay for $15). It has every wire and vacuum line in easy to follow diagrams. I am an automation engineer, so I could have figured it out, but the book made short work of it. I will not do another swap such as this without the shop manual - it saved TONS of time in tracing things. I was also careful to mark everything during disassembly.
Mistakes made - I cut away too much of the original dash, ended up welding most of it back in to use for mounting the vic dash. I was too careless with the AC lines and now need to get another set to modify for use, the ones I removed were too manhandled to use. Double up on the firewall where the master cylinder bolts up, with the angle and length of the pedal, there is a lot of force placed on the master cylinder mount causing movement - followed by cursing - followed by removal and subsequent proper bracing.
The car drives spectacular, excellent ride. I thought I would regret not using a 4 cam 4.6, but as it turns out, the sohc motor has plenty of power after all.
Hope this helps.
JD