1970 Mach 1

Hello, just wanting to get a couple opinions on a fair price on a 1970 Mach 1 a inlaw has offered to sell me.It is a very nice car that has absolutely no rust,dings or scratches and a 15 year old paint job that still looks perfect due to the car being garage kept.The grabber blue paint job and the shaker hood definally still gets your attention.There are a few issues with the car though, the owner decided to drag race the car and pulled out the seats and carpet and installed a 9 point cage which i would definally get removed.The door panels as well as the back seat area are all done nicely in sheet aluminum.The dash is in great condition as well as all the factory guages.The radio is missing and i do not know how much of the air conditioning remains.The reason the car was parked is the transmission went out and was removed so the c6 transmission is missing as well as the driveshaft.Not sure what happened to these.Also the car was originally a 351 car but my inlaw installed a 428 cobra jet motor in it so the car is not completely original but still nice I beleive with the big block installed.The 428 came out of a mustang from my understanding and looks great under the hood.Factory vented valve covers, accessories, and internals from what I have been told.Supposedly the motor ran good when the car was parked and being an inlaw with no pressure to sell the car besides to clear garage space I know he would be honest about that.Thankfully he is not selling the car to try to make much money and has offered it to me for what I think is a great price but was just wondering what you guys think a good price would be with the things that need to be done as well as the motor not being original.I have plans on selling my Trans Am to help pay for the work that I will need done.It's not everyday that i see a classic in this great of condition and I dont think I can pass on the deal.Thanks for any opinions.