I can understand you there Justin ( on the "shell" situation). I know what you mean in a build progress too. Honestly, the best way to do it IMHO is if you have engine rebuild stuff you need to do. Get that all done during the winter months. I did my build during the winter, did all my sandblasting of spare parts I had (mounts, etc), did all my idea formation...then swapped in the spring. Took my time on the motor build and built as $$$ allowed at the time, etc. and its still a continuing work in progress. I thought maybe "paint" this year...well, that may or may not happen.
My idea has been that with the setup I've done with the car, I wanted to go through and make sure I had everything mechnically setup the way I wanted to... i.e. intercooler mounting, pump locations, brackets that I'd have to drill holes for, etc. etc. so that when I went for final paint and body...all non-needed holes, etc. would be covered up, rewelded, etc. and then later on once it is painted...I'm not drilling or making holes in good fresh paint!
So...she kinda goes "rat stylz" at the moment. Or OEM -worn
The pics do make it tight. You also have to decide...is this a car you'll be driving year round once the swap is done or not? In the "not" situation, you could remove the raintray from the car and open up the bay a bit and still retaining the heater, etc. by building in a close box for it (i'm doing this , this winter)...but I wouldn't recommend removal of the raintray for year round use.
Also, on your MKIV dash swap...it has been done, and can be. There are threads on the vortex detailing it. Its ALOT of cutting with the MKIV dash and most recover them with something else in the process as well. There was also a guy parting a partially finished 1.8T rabbit on there as well, and it had a modified MKIV dash...not completely finished, but it was partly done. There are considerations when you do the MKIV dash swap about what heating setup you'd use, which involves relocating the blower box, all accessories, some cutting, welding, etc. etc.
A different story though if it isn't a year round car.
Another solution for that is what Mr.Dave did. He use the stock MK1 dash and made his own bezel for the MKIV cluster goodies, etc. and it turned out fairly nice as well!
As for your brakes being soft. Check that master cylinder...they can go bad without loosing fluid...i had mine go on the b3 here this past spring and it was a "soft brake" problem.
Doesn't sound like its the booster...but, you never know...
Vac. pump diaphrapm been replaced lately as well? Always nice in making sure the brake system is healthy
Joe