1963 Plymouth Valiant Signet Convertible Restoration...HELLLP!!!

I am finally moving forward on a budget restoration of my 63 Valiant Convertible. My original engine (Slant Six 170) spun a bearing and I can not find a bottom rebuild kit for it. This car came with the 170, which wouldn't pull the hat off your head, and a Push Button 904 Torqueflight automatic transmission.
The top has dry rotted at the mounts near the side back windows. I am currently budgeting that out and it appears to be going ok; however, I have a 318 with a 727 transmission that came out of a 76 D100 PU. I have a shop doing a budget building the engine for me.
Unfortunately, I have no idea what the hell I am doing...and I admit my shortcomings. This is why I joined here hoping that someone who has done this restoration or one like it can help me.
Questions:
Can I fit the 318 into my Valiant?
Can I put headers on the 318
Can I use my 904..although the bell housing is for a Slant 6 or do I have to use the 727?
I would like to keep my push buttons...can I do this?
Do I have to change the drive shaft?
Can I use the stock rear end?
Please respond if you feel like it and if you have knowledge on this restoration...Thank You

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This has been discussed here before, but the 63 A body has a different fire wall that might interfere with the distributor on a V-8. You may also
be better off using a 904, because a 727 might have clearance problems. To use a push button trans. you would have to use a 65 or older cable
operated trans. 66 up trans. is a slip yoke, lever operated trans.