On the frame thing, to add to above, the comments about the engine placement can be very true even for the most common of engines. Planning for firewall clearance based on distributor diameter, water pump length, exhaust clearance to steering box, on and on details can lead to compromises by the manufacturer that might not suit your tastes/needs. Then there are some unexpected things. In years past, though not so in the last 10, I knew of three different people who had problems with TCI chassis not having body mount holes, engine mounts, and steering box brackets in the correct place. These things are supposedly jig built, by a supposed reputable big name supplier, but for whatever reason they needed to be reworked. Probably has happened with other builders, just haven't heard about it. This part of the hobby is entering a gradual decline in participation as we older guys are the core and many are leaving the building. Various long time, and less, suppliers are exiting the market. It wouldn't be a surprise if, as they wind down, quality control were to slip. Not all doom and gloom, just a reason to be even more cautious than ever before.